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LIBRARY 

OF  THE 

UNIVERSITY  OF,  CALIFORNIA. 

Class 


THE 
LIBRARY   OF    USEFUL  STORIES 


FIG.  i.— THE  PLANET  SATURN. 


THE  STORY 
OF  THE   SOLAR  SYSTEM 


BY 

GEORGE   F.   CHAMBERS,    F.  R.  A.  S. 

OF    THE    INNER    TEMPLE,    BARRISTbR-AT-LAW 

AUTHOR    OF    THE    STORY    OF    THE    STARS 


WITH 


^     OF  THE 

UNIVERSITY 

OF 


NEW     YORK 

D.    APPLETON    AND    COMPANY 
1907 


COPYRIGHT,  1895, 
BY  D.   APPLETON   AND   COMPANY, 


PREFACE. 


HAVING  in  my  "  Story  of  the  Stars  "  told  of 
far  distant  suns,  many  of  them  probably  with 
planets  revolving  around  them,  I  have  in  the 
present  volume,  which  is  a  companion  to  the 
former  one,  to  treat  of  the  Sun  in  particular — our 
Sun  as  we  may  call  him — and  the  body  of  attend- 
ants which  own  his  sway  by  revolving  round  him. 
The  attendants  are  the  planets,  commonly  so 
called,  together  with  a  certain  number  of  comets. 
I  shall  deal  with  all  these  objects  rather  from  a 
descriptive  and  practical  than  from  a  speculative 
or  essay  point  of  view,'  and  with  special  reference 
to  the  convenience  and  opportunities  of  persons 
possessing,  or  having  access  to,  what  may  be 
called  popular  telescopes — telescopes  say  of  from 
two  to  four  inches  of  aperture,  and  costing  any 
sum  between  ^10  and  £$o.  There  is  much 
pleasure  and  profit  to  be  got  out  of  telescopes  of 
this  type,  always  presuming  that  they  are  used  by 
persons  possessed  of  patience  and  perseverance. 
It  is  a  very  great  mistake,  though  an  extremely 
common  one,  to  suppose  that  unless  a  man  can 
command  a  big  telescope  he  can  cjo  no  useful 

193715 


4  PREFACE. 

work,  and  derive  no  pleasure  from  his  work.  To 
all  such  croakers  I  always  point  as  a  moral  the 
achievements  of  Hermann  Goldschmidt,  who  from 
an  attic  window  at  Fontenay-aux-Roses  near 
Paris,  with  a  telescope  of  only  2\  inches  aperture, 
discovered  no  fewer  than  14  minor  planets. 

As  this  volume  is  intended  for  general  read- 
ing, rather  than  for  educational  or  technical  pur- 
poses, I  have  kept  statistical  details  and  numer- 
ical expressions  within  very  narrow  limits,  mere 
figures  being  always  more  or  less  unattractive. 

John  Richard  Green,  in  the  Preface  to  his 
book  on  The  Making  of  England,  writes  as  fol- 
lows: — "I  may  add,  in  explanation  of  the  re- 
appearance of  a  few  passages  .  .  .  which  my 
readers  may  have  seen  before,  that  v  here  I  had 
little  or  nothing  to  add  or  to  change,  I  have  pre- 
ferred to  insert  a  passage  from  previous  work, 
with  the  requisite  connections  and  recerences,  to 
the  affectation  of  rewriting  such  a  passage  for  the 
mere  sake  of  giving  it  an  air  of  novelty."  I  will 
venture  to  adopt  this  thought  as  my  own,  and  to 
apply  it  to  the  repetition,  here  and  there,  of  ideas 
and  phrases  which  are  already  to  be  found  in  my 
Handbook  of  Astronomy. 

G.  F.  C. 

NORTHFIELD  GRANGE, 

EASTBOURNE,  1895. 


CONTENTS. 


CHAPTER  PAGE 

I.  INTRODUCTORY  STATEMENT        ....        7 

II.  THE  SUN 18 

III.  MERCURY     .        .        .    ; ',. 'V .  .        .       .        .57 

•  IV.  VENUS  .        .        .       .        .        .       .        .        .61 

V.  THE  EARTH.        .        .        .  »  .  .  .69 

•  VI.  THE  MOON  .        .        •  '.-*.  •  .  .  .      89 
.VII.  MARS    .        .        .        »        .  *  .  .  .100 
VIII.  THE  MINOR  PLANETS.        .  .  •  .  .  .    no 
»  IX.  JUPITER        .        ..      ,  ".  Y  .  .  .  .     115 

•  X.  SATURN      .»       >v's    ;Y     .    ^  •        •        •        .122 

•  XI.  URANUS        .        .        ,        .        .        .        .        .  138 

*XII.  NEPTUNE 143 

XIII.  COMETS 150 

APPENDIX — TABLES  OF  THE  SOLAR  SYSTEM        .        .  182 

GENERAL  INDEX     .        .        .        .        .        .        .        .  185 


LIST   OF   ILLUSTRATIONS. 


FIGURE  PAGE 

1.  The  Planet  Saturn Frontispiece 

2.  Inclination  of  Planetary  Orbits  .....  9 

3.  Comparative  Sizes  of  Major  Planets  .         .         .  II 

4.  Comparative  Size  of  the  Sun  as  seen  from  the  Planets 

Named  .         .   -  .         .         .         .         .         .17 

5.  Ordinary  Sun-spots,  June  22,  1885    ....  22 

6.  Change  of  Form  in  Sun-spots  Owing  to  the  Sun's 

Rotation         ........  29 

7.  Sun-spots  seen  as  a  Notch          .....  37 

8.  The  Sun  Totally  Eclipsed,  July  18,  1860  ...  56 

9.  Venus,  Dec.  23,  1885 64 

10.  Venus  Near  Conjunction  as  a  Thin  Crescent     .         .  65 

ir.  Mare  Crisium  (Lick  Observatory  photographs)          .  90 

12.  Four  Views  of  Mars  (Barnard)  .....  101 

13.  Mars,  Aug.  27,  1892  (Guyot) 107 

14.  Jupiter,  Nov.  27,  1857  (Dawes)          .         .         .        .116 

15.  Saturn,  1889 11:3 

16.  General  View  of  the  Phases  of  Saturn's  Rings   .         .  126 

17.  Phases  of  Saturn's  Rings  at  Specified  Dates      .         .  129 

18.  Saturn  with  Titan  and  its  Shadow     ....  137 

19.  Telescopic  Comet  with  a  Nucleus     ....  154 

20.  Comet  seen  in  Daylight,  Sept.,  1882           .         .         .  155 

21.  Quenisset's  Comet,  July  9,  1893         ....  156 

22.  Holmes's  Comet,   the  Head  on  Nov.  9,  1892  (Den- 

ning)       159 

23.  Holmes's  Comet,  the  Head  on  Nov.  16,  1892  (Den- 

ning)               ...  159 

24.  Comet  III.  of  1862,  on  Aug.   22,  showing   Jet  of 

Luminous  Matter  (Challis) 160 

25.  Sawerthal's  Comet,  June  4,  1888  (Charlois)       .         .  165 

26.  Biela's  Comet,  1846 169 

27.  The  Great  Comet  of  1811 177 

28.  The  Great  Comet  of  1882 179 

6 


THE 
STORY  OF  THE  SOLAR  SYSTEM. 

CHAPTER   I. 

INTRODUCTORY    STATEMENT. 

BY  the  term  "  Solar  System  "  it  is  to  be  under- 
stood that  an  Astronomer,  speaking  from  the 
standpoint  of  an  inhabitant  of  the  Earth,  wishes 
to  refer  to  that  object,  the  Sun,  which  is  to  him 
the  material  and  visible  centre  of  life  and  heat 
and  control,  and  also  to  those  bodies  dependent 
on  the  Sun  which  circulate  round  it  at  various 
distances,  deriving  their  light  and  heat  from  the 
Sun,  and  known  as  planets  and  comets.  The 
statement  just  made  may  be  regarded  as  a  general 
truth,  but  as  the  strictest  accuracy  on  scientific 
matters  is  of  the  utmost  importance,  a  trivial 
reservation  must  perhaps  be  put  upon  the  fore- 
going broad  assertion.  There  is  some  reason  for 
thinking  that  possibly  one  of  the  planets  (Jupiter) 
possesses  a  little  inherent  light  of  its  own  which 
is  not  borrowed  from  the  Sun ;  whilst  of  the 
comets  it  must  certainly  be  said  that,  as  a  rule, 
they  shine  with  intrinsic,  not  borrowed  light.  Re- 
specting these  reservations  more  hereafter. 

The  planets  are  divided  into  "  primary  "  and 
"secondary."  By  a  "  primary  "  planet  we  mean 
one  which  directly  circulates  round  the  Sun  ;  by 
7 


8  THE  STORY  OF  THE  SOLAR  SYSTEM. 

a  "  secondary  "  planet  we  mean  one  which  in  the 
first  instance  circulates  round  a  primary  planet, 
and  therefore  only  in  a  secondary  sense  circulates 
round  the  Sun.  The  planets  are  also  "  major  " 
or  "  minor  " ;  this,  however,  is  only  a  distinction 
of  size. 

The  secondary  planets  are  usually  termed 
"  satellites,"  or,  very  often,  in  popular  language, 
"  moons,"  because  they  own  allegiance  to  their 
respective  primaries  just  as  our  Moon — the  Moon 
—does  to  the  Earth.  But  the  use  of  the  term 
"moon  "  is  inconvenient,  and  it  is  better  to  stick 
to  "satellite." 

There  is  yet  another  method  of  classifying  the 
planets  which  has  its  advantages.  They  are  some- 
times divided  into  "inferior"  and  "superior." 
The  "inferior"  planets  are  those  which  travel 
round  the  Sun  in  orbits  which  are  inside  the 
Earths  orbit;  the  "  superior  "  planets  are  those 
whose  orbits  are  outside  the  Earth. 

The  following  is  an  enumeration  of  the  major 
planets  in  the  order  of  their  distances,  reckoning 
from  the  Sun  outwards : — 

1.  Mercury. 

2.  Venus. 

3.  The  Earth. 

4.  Mars. 

5.  Jupiter. 

6.  Saturn. 

7.  Uranus. 

8.  Neptune. 

All  the  above  are  major  planets  and  also  primary 
planets.  In  between  Nos.  4  and  5  circulate  the 
"  Minor  "  planets,  an  ever-increasing  body,  now 
more  than  400  in  number,  but  all,  except  one  or 
perhaps  two,  invisible  to  the  naked  eye. 


INTRODUCTORY  STATEMENT. 


The  "  Inferior  " 
planets  it  will  be 
seen  from  the 
above  table  com- 
prise Mercury  and 
Venus,  whilst  the 
"  Superior  "planets 
are  Mars  and  all 
those  beyond. 

Great  differ- 
ences exist  in  the 
inclinations  of  the 
orbits  of  the  differ- 
ent planets  to  the 
plane  of  the  eclip- 
tic, a  fact  which  is 
better  shown  by  a 
diagram  than  by  a 
table  of  mere  fig- 
ures. The  orbit  of 
Uranus  is  indeed 
so  much  inclined 
that  its  motion  is 
really  retrograde 
compared  with  the 
general  run  of  the  _ 
planets  :  and  the 
same  remark  ap- 
plies, though  much 
more  forcibly,  to 
the  case  of  Nep- 
tune. 

The  actual 

movements  of  the 
planets  round  the 
Sun  are  extremely 


10          THE  STORY  OF  THE  SOLAR  SYSTEM. 

simple,  for  they  do  nought  else  but  go  on,  and  on, 
and  on,  incessantly,  always  in  the  same  direction, 
and  almost,  though  not  quite,  at  a  uniform  pace, 
though  in  orbits  very  variously  inclined  to  the 
plane  of  the  ecliptic.  But  an  element  of  extreme 
complication  is  introduced  into  their  apparent 
movements  by  reason  of  the  fact  that  we  are 
obliged  to  study  the  planets  from  one  of  their  own 
number,  which  is  itself  always  in  motion. 

If  the  Earth  itself  were  a  fixture,  the  study  of 
the  movements  of  the  planets  would  be  a  com- 
paratively easy  matter,  whilst  to  an  observer  on 
the  Sun  it  would  be  a  supremely  easy  matter. 

Greatly  as  the  planets  differ  among  themselves 
in  their  sizes,  distances  from  the  Sun,  and  physical 
peculiarities,  they  have  certain  things  in  common, 
and  it  will  be  well  to  make  this  matter  clear  be- 
fore we  go  into  more  recondite  topics.  For  in- 
stance, not  only  do  they  move  incessantly  round 
the  Sun  in  the  same  direction  at  a  nearly  uniform 
pace,  but  the  planes  of  their  orbits  are  very  little 
inclined  to  the  common  plane  of  reference,  the 
ecliptic,  or  to  one  another.*  The  direction  of 
motion  of  the  planets  as  viewed  from  the  north 
side  of  the  ecliptic  is  contrary  to  the  motion  of 
the  hands  of  a  watch.  Their  orbits,  unlike  the 
orbits  of  comets,  are  nearly  circular,  that  is,  they 
are  only  very  slightly  oval.  Agreeably  to  the 
principles  of  what  is  known  as  the  Law  of  Uni- 
versal Gravitation,  the  speed  with  which  they 
move  in  their  orbits  is  greatest  in  those  parts 


*  The  remark  in  the  text  applies  to  all  the  major  planets 
and  to  a  large  number  of  the  minor  planets,  but  certain  of  the 
minor  planets  travel  in  orbits  which  are  considerably  inclined 
to  the  ecliptic,  and  therefore  to  ajl  the  other  planets. 


INTRODUCTORY  STATEMENT.        II 

which  lie  nearest  the  Sun,  and  least  in  those  parts 
which  are  most  remote  from  the  Sun  ;  in  other 


FIG.  3. — Comparative  Sizes  of  the  Major  Planets. 


words,    they    move    quickest    in    Perihelion    and 
slowest  in  Aphelion. 

The  physical  peculiarities  which  the  planets 
have  in  common  include  the  following  points  : — 
they  are  opaque  bodies,  and  shine  by  reflecting 
light  which  they  receive  from  the  Sun.  Probably 


12          THE  STORY  OF  THE  SOLAR  SYSTEM. 

all  of  them  are  endued  with  an  axial  rotation, 
hence  their  inhabitants,  if  there  are  any,  have  the 
alternation  of  day  and  night,  like  the  inhabitants 
of  the  Earth,  but  the  duration  of  their  days, 
measured  in  absolute  terrestrial  hours,  will  in 
most  cases  differ  materially  from  the  days  and 
nights  with  which  we  are  familiar. 

I  stated  on  a  previous  page  that,  owing  to  the 
circumstances  in  which  we  find  ourselves  on  the 
Earth,  the  apparent  and  real  movements  of  the 
planets  are  widely  different.  It  would  be  beyond 
the  scope  of  this  little  work  to  go  into  these 
differences  in  any  considerable  detail;  suffice  it 
then  to  indicate  only  a  few  general  points.  In 
the  first  place,  an  important  distinction  exists  be- 
tween the  visible  movements  of  the  inferior  and 
superior  planets.  The  inferior  planets,  Mercury 
and  Venus,  lying  as  they  do  within  the  orbit  of 
the  Earth,  are  much  restricted  in  their  movements 
in  the  sky.  We  can  never  see  them  except  when 
they  are  more  or  less  near  to  the  rising  (or  risen) 
or  setting  (or  set)  Sun.  The  extreme  angular 
distance  from  the  Sun  in  the  sky  to  which  Mer- 
cury can  attain  is  but  27°,  and  therefore  we  can 
never  observe  it  otherwise  than  in  sunliglit  or 
twilight,  for  it  never  rises  more  than  i£  hours 
before  sunrise  nor  sets  later  than  i£  hours  after 
sunset.  Of  course  between  these  limits  the  planet 
is  above  the  horizon  all  the  time  that  the  Sun  is 
above  the  horizon,  but  except  in  very  large  tele- 
scopes is  not  usually  to  be  detected  during  the 
day-time.  These  remarks  regarding  Mercury 
apply  likewise  in  principle  to  Venus;  only  the 
orbit  of  Venus  being  larger  than  the  orbit  of 
Mercury,  and  Venus  itself  being  larger  in  size 
than  Mercury,  the  application  of  these  principles 


INTRODUCTORY  STATEMENT.        13 

leads  to  somewhat  different  results.  The  greatest 
possible  distance  of  Venus  may  be  47°  instead  of 
Mercury's  27°.  Venus  is  therefore  somewhat 
more  emancipated  from  the  effects  of  twilight. 
The  body  of  Venus  being  also  very  much  larger 
and  brighter  than  the  body  of  Mercury,  it  may  be 
more  often  and  more  easily  detected  in  broad  day- 
light. 

It  follows  from  the  foregoing  statement  that 
the  inferior  planets  can  never  be  seen  in  those 
regions  of  the  heavens  which  are,  as  it  is  technic- 
ally called,  in  "  Opposition  "  to  the  Sun  ;  that  is, 
which  are  on  the  meridian  at  midnight  whilst  the 
Sun  is  on  the  meridian  in  its  midday  splendour  to 
places  on  the  opposite  side  of  the  Earth.  On  the 
other  hand,  the  two  inferior  planets  on  stated, 
though  rare,  occasions  exhibit  to  a  terrestrial 
spectator  certain  phenomena  of  great  interest  and 
importance  in  which  no  superior  planet  can  ever 
take  part.  I  am  here  referring  to  the  "  Transits  " 
of  Mercury  and  Venus  across  the  Sun.  If  these 
planets  and  the  Earth  all  revolved  round  the  Sun 
exactly  in  the  plane  of  the  ecliptic,  transits  of 
these  planets  would  be  perpetually  recurring  after 
even  intervals  of  only  a  few  months;  but  the  fact 
that  the  orbit  of  Mercury  is  inclined  7°,  and  that 
of  Venus  about  3^,  to  the  ecliptic,  involves  such 
complications  that  transits  of  Mercury  only  occur 
at  unequal  intervals  of  several  years,  whilst,  in 
extreme  cases,  more  than  a  century  may  elapse 
between  two  successive  transits  of  Venus.  For  a 
transit  of  an  inferior  planet  over  the  Sun  to  take 
place,  the  Earth  and  the  planet  and  the  Sun  must 
be  exactly  in  the  same  straight  line,  reckoned  both 
vertically  and  horizontally.  Twice  in  every  revo- 
lution round  the  Sun  an  inferior  planet  is  verti- 


14  THE  STORY  OF  THE   SOLAR  SYSTEM. 

cally  in  the  same  straight  line  with  the  Earth  and 
the  Sun  ;  and  it  is  said  to  be  in  "  inferior  conjunc- 
tion "  when  the  planet  comes  between  the  Earth 
and  the  Sun  ;  and  in  "  superior  conjunction  "  when 
the  planet  is  on  the  further  side  of  the  Sun,  the 
Sun  intervening  between  the  Earth  and  the  planet. 
But  for  all  three  to  be  horizontally  in  the  same 
straight  line  is  quite  another  matter.  It  is  the 
orbital  inclinations  of  Mercury  and  Venus  which 
enable  them,  so  to  speak,  to  dodge  an  observer 
who  is  on  the  lookout  to  see  them  pass  exactly  in 
front  of  the  Sun,  or  to  disappear  behind  the  Sun  ; 
and  so  it  comes  about  that  a  favourable  combina- 
tion of  circumstances  which  is  rare  is  needed  be- 
fore either  of  the  aforesaid  planets  can  be  seen 
as  round  black  spots  passing  in  front  of  the  Sun. 
A  passage  of  either  of  these  planets  behind  the 
Sun  could  never  be  seen  by  human  eye,  because 
of  the  overpowering  brilliancy  of  the  Sun's  rays, 
even  though  an  Astronomer  might  know  by  his 
calculations  the  exact  moment  that  the  planet 
was  going  to  pass  behind  the  Sun. 

When  an  inferior  planet  attains  its  greatest 
angular  distance  from  the  Sun,  as  we  see  it  (which 
I  have  already  stated  to  be  about  27°  in  the  case 
of  Mercury  and  47°  in  the  case  of  Venus),  such 
planet  is  said  to  be  at  its  "  greatest  elongation," 
"  east  "  or  "  west,"  as  the  case  may  be.  At  east- 
ern elongation  or  indeed  whenever  the  planet  is 
east  of  the  Sun,  it  is,  to  use  a  familiar  phrase,  an 
"evening  star";  on  the  other  hand,  at  western 
elongation,  or  whenever  it  is  on  the  western  side 
of  the  Sun,  it  is  known  as  a  "  morning  star." 

If  the  movements  of  an  inferior  planet  are  fol- 
lowed sufficiently  long  by  the  aid  of  a  star  map,  it 
will  be  seen  that  sometimes  it  appears  to  be  pro- 


INTRODUCTORY  STATEMENT.        15 

ceeding  in  a  forward  direction  through  the  signs 
of  the  zodiac;  then  for  a  while  it  will  seem  to 
stand  still ;  then  at  another  time  it  will  apparently 
go  backwards,  or  possess  a  retrograde  motion. 
All  these  peculiarities  have  their  originating  cause 
in  the  motion  of  the  Earth  itself,  for  the  absolute 
movement  of  the  planet  never  varies,  being  always 
in  the  same  direction,  that  is,  forwards  in  the  order 
of  the  signs. 

Turning  now  to  the  superior  planets,  we  have 
to  face  an  altogether  different  succession  of  cir- 
cumstances. A  superior  planet  is  not,  as  it  were, 
chained  to  the  Sun  so  as  to  be  unable  to  escape 
beyond  the  limits  of  morning  or  evening  twilight ; 
it  may  have  any  angular  distance  from  the  Sun 
up  to  180°,  reaching  which  point  it  approaches 
the  Sun  on  the  opposite  side,  step  by  step,  until 
it  again  comes  into  conjunction  with  the  Sun. 
As  applied  to  a  superior  planet,  the  term  "con- 
junction "  means  the  absolute  moment  when  the 
Earth  and  the  Sun  and  the  planet  are  in  the  same 
straight  line,  the  Sun  being  in  the  middle.  In 
such  a  case,  to  us  on  the  Earth  the  planet  is  lost 
in  the  Sun's  rays,  whilst  to  a  spectator  on  the 
planet  the  Earth  would  appear  similarly  lost  in 
the  Sun's  rays,  as  the  Earth  would  be  at  that  stage 
of  her  orbit  which  we,  speaking  of  our  inferior 
planets,  call  superior  conjunction. 

For  a  clear  comprehension  of  all  the  various 
matters  which  we  have  just  been  speaking  of, 
a  careful  study  of  diagrams  of  a  geometrical 
character,  or  better  still,  of  models,  would  be 
necessary. 

Something  must  now  be  said  about  the  phases 
of  the  planets.  Mercury  and  Venus,  in  regard  to 
their  orbital  motions,  stand  very  much  on  the 
2 


1 6  THE  STORY  OF  THE  SOLAR  SYSTEM. 

same  footing  with  respect  to  the  inhabitants  of 
the  Earth  as  the  Moon  does,  and  accordingly  both 
those  planets  in  their  periodical  circuits  round 
the  Sun  exhibit  the  same  succession  of  phases  as 
the  Moon  does.  In  the  case,  however,  of  the 
superior  planets  things  are  otherwise.  Two  only 
of  them,  Mars  and  Jupiter,  are  sufficiently  near 
the  Earth  to  exhibit  any  phase  at  "all.  When  they 
are  in  quadrature  (/.  *.,  90°  from  the  Sun  on  either 
side)  there  is  a  slight  loss  of  light  to  be  noticed 
along  one  limb.  In  other  words,  the  disc  of  each 
ceases  for  a  short  time,  and  to  a  slight  extent,  to 
be  truly  circular;  it  becomes  what  is  known  as 
"gibbous."  This  occasional  feature  of  Mars  may 
be  fairly  conspicuous,  or,  at  least,  noticeable;  but 
in  the  case  of  Jupiter  it  will  be  less  obvious  un- 
less a  telescope  of  some  size  is  employed. 

If  the  major  planets  are  arbitrarily  ranged  in 
two  groups,  Mercury,  Venus,  the  Earth  and  Mars 
being  taken  as  an  interior  group,  comparatively 
near  the  Sun  ;  whilst  Jupiter,  Saturn,  Uranus  and 
Neptune  are  regarded  as  an  exterior  group,  being 
at  a  great  distance  from  the  Sun,  it  will  be  found 
that  some  important  physical  differences  exist 
between  the  two  groups. 

Of  the  interior  planets,  the  Earth  and  Mars 
alone  have  satellites,  and  between  them  make  up 
a  total  of  only  three.  The  exterior  planets,  on 
the  other  hand,  all  have  satellites,  the  total  num- 
ber being  certainly  seventeen,  and  possibly  eight- 
een. In  detail,  Jupiter  has  four,  Saturn  eight, 
Uranus  four,  and  Neptune  one,  and  perhaps  two. 
These  facts  may  be  regarded  as  an  instance  of 
the  beneficence  of  the  Creator  of  the  Universe  if 
we  consider  that  the  satellites  of  these  remoter 
planets  are  so  numerous,  in  order  that  by  their 


INTRODUCTORY  STATEMENT.  '      17 

numbers  they  may  do  something  to  make  up  for 
the  small  amount  of  light  which,  owing  to  their 
distance  from  the  Sun,  their  primaries  receive. 
Then  again,  the  average  density  of  the  first  group 


FIG.  4. — Comparative  size  of  the  Sun  as  seen  from  the  Planets 
named. 

of  planets  greatly  exceeds  the  average  density  of 
the  second  group  in  the -approximate  ratio  of  5 
to  i.  Finally,  there  is  reason  to  believe  that 
a  marked  difference  exists  in  the  axial  rotations 
of  the  planets  forming  the  two  groups.  We  do 
not  know  the  precise  figures  for  all  the  exterior 
planets,  but  the  knowledge  which  we  do  possess 
seems  to  imply  that  the  average  length  of  the  day 
in  the  case  of  the  interior  planets  is  about  twenty- 
four  hours,  but  that  in  the  case  of  the  exterior 
planets  it  is  no  more  than  about  ten  hours.  These 
figures  can,  however,  only  be  presented  as  pos- 
sibly true,  because  observations  on  the  rotation 


1 8        THE  STORY  OF  THE  SOLAR  SYSTEM. 

periods  of  Mercury  and  Venus  on  the  one  hand, 
and  of  Uranus  and  Neptune  on  the  other,  are  at- 
tended with  so  much  difficulty  that  the  recorded 
results  are  of  doubtful  trustworthiness.  It  is, 
however,  reasonable  to  presume  that  the  actual 
size  of  the  respective  planets  has  more  to  do  with 
the  matter  than  their  distances  from  the  Sun. 

I  think  that  the  foregoing  summary  respecting 
the  planets  collectively  embraces  as  many  points 
as  are  likely  to  be  of  interest  to  the  generality 
of  readers;  we  will  therefore  pass  on  to  consider 
somewhat  in  detail  the  several  constituent  mem- 
bers of  the  solar  system,  beginning  with  the  Sun. 


CHAPTER   II. 

THE    SUN. 

THERE  was  once  a  book  published,  the  title  of 
which  was  "  The  Sun,  Ruler,  Fire,  Light  and  Life 
of  the  Planetary  System."  The  title  was  by  no 
means  a  bad  one,  for  without  doubt  the  Sun  may 
fairly  be  said  to  represent  practically  all  the  ideas 
conveyed  by  the  designations  quoted. 

There  is  certainly  no  one  body  in  creation 
which  is  so  emphatically  pre-eminent  as  the  Sun. 
Whether  or  no  there  are  stars  which  are  suns — 
centres  of  systems  serving  in  their  degree  the 
purposes  served  by  our  Sun,  I  need  not  now  pause 
to  enquire,  though  I  think  the  idea  is  a  very 
probable  one ;  but  of  those  celestial  objects  with 
which  our  Earth  has  a  direct  relationship,  beyond 
doubt  the  Sun  is  unquestionably  entitled  to  the 
foremost  place.  It  is,  as  it  were,  the  pivot  on 


THE   SUN.  19 

which  the  Earth  and  all  the  various  bodies  com- 
prising the  Solar  System  revolve  in  their  annual 
progress.  It  is  our  source  of  light  and  heat,  and 
therefore  may  be  called  the  great  agent  by  which 
an  Almighty  Providence  wills  to  sustain  animal 
and  vegetable  life.  The  consideration  of  all  the 
complicated  questions  which  arise  out  of  these 
functions  of  the  Sun  belongs  to  the  domain  of 
Physics  rather  than  that  of  Astronomy  ;  still  these 
matters  are  of  such  momentous  interest  that  an 
allusion  to  them  must  be  made,  for  they  ought 
not  to  be  lost  sight  of  by  the  student  of  Astron- 
omy. Half  a  century  ago  the  actual  state  of 
our  knowledge  respecting  the  Sun  might  without 
difficulty  be  brought  within  the  compass  of  a  single 
chapter  in  any  book  on  Astronomy,  but  so  enor- 
mous has  been  the  development  of  knowledge 
respecting  the  Sun  of  late  years,  that  it  is  no 
longer  a  question  of  getting  the  materials  prop- 
erly into  one  chapter,  but  it  is  a  matter  of  a 
whole  volume  being  devoted  to  the  Sun,  or  even, 
as  in  the  case  of  Secchi,  of  two  large  octavo  vol- 
umes of  500  pages  each  being  required  to  cover 
the  whole  ground  exhaustively.  The  reader  will 
therefore  easily  understand  that  in  the  space  at 
my  disposal  in  this  little  work  nothing  but  a  pass- 
ing glimpse  can  possibly  be  obtained  of  this  great 
subject.  It  is  great  not  only  in  regard  to  the  vast 
array  of  purely  astronomical  facts  which  are  at  a 
writer's  command,  but  also  on  account  of  the  ex- 
tensive ramifications  which  the  subject  has  into 
the  domains  of  chemistry,  photography,  optics 
and  cognate  sciences.  I  shall  therefore  endeavour 
to  limit  myself  generally  to  what  an  amateur  can 
see  for  himself  with  a  small  telescope,  and  can 
readily  understand,  rather  than  attempt  to  say  a 


20  THE  STORY   OF  THE   SOLAR   SYSTEM. 

little  something  about  everything,  and  fail  in  the 
effort. 

The  mean  distance  of  the  Earth  from  the  Sun 
may  be  taken  to  be  about  93  millions  of  miles, 
and  this  distance  is  employed  by  Astronomers  as 
the  unit  by  which  most  other  long  celestial  dis- 
tances are  reckoned.  The  true  diameter  of  the 
Sun  is  about  866,000  miles.  The  surface  area  ex- 
ceeds that  of  the  Earth  11,946  times,  and  the  vol- 
ume is  1,305,000  times  greater.  The  mass  or 
weight  of  the  Sun  is  332,000  times  that  of  the 
Earth,  or  about  700  times  that  of  all  the  planets 
put  together.  Bulk  for  bulk  the  Sun  is  much 
lighter  than  the  Earth :  whilst  a  cubic  foot  of  the 
Earth  on  an  average  weighs  rather  more  than  5 
times  as  much  as  a  cubic  foot  of  water,  a  cubic 
foot  of  Sun  is  only  about  3^  times  the  weight  of 
the  same  bulk  of  water.  This  consideration  of 
the  comparative  lightness  of  the  Sun  (though  in 
his  day  the  Sun  was  thought  to  be  lighter  than  it 
is  now  supposed  to  be)  led  Sir  J.  Herschel  to  in- 
fer that  an  intense  heat  prevails  in  its  interior, 
independent  it  may  be  of  its  surface  heat,  so  to 
speak,  of  which  alone  we  are  directly  cognizant 
by  the  evidence  of  our  senses. 

The  Sun  is  a  sphere,  and  is  surrounded  by  an 
extensive  but  attenuated  envelope,  or  rather  series 
of  envelopes,  which  taken  together  bear  some 
analogy  to  the  atmosphere  surrounding  the  Earth. 
These  envelopes,  which  we  shall  have  to  consider 
more  in  detail  presently,  throw  out  rays  of  light 
and  heat  to  the  confines  of  the  Solar  System, 
though  as  to  the  conditions  and  circumstances 
under  which  that  light  and  heat  are  generated  we 
are  entirely  ignorant.  Of  the  potency  of  the 
Sun's  rays  we  can  form  but  a  feeble  conception, 


THE  SUN.  21 

for  the  amount  received  by  the  Earth  is,  it  has 
been  calculated,  but  one  23oo-millionth  of  the 
whole.  Our  annual  share  would,  it  is  supposed, 
be  sufficient  to  melt  a  layer  of  ice  spread  uniform- 
ly over  the  Earth  to  a  depth  of  100  feet,  or  to 
heat  an  ocean  of  fresh  water  60  feet  deep  from 
freezing  point  to  boiling  point.  The  illuminating 
power  of  the  Sun  has  to  be  expressed  in  language 
of  similar  profundity.  Thus  it  has  been  calcu- 
lated to  equal  that  which  would  be  afforded  by 
5563  wax  candles  concentrated  at  a  distance  of 
one  foot  from  the  observer.  Again,  it  has  been 
concluded  that  no  fewer  than  half  a  million  of 
full  moons  shining  all  at  once  would  be  required 
to  make  up  a  mass  of  light  equal  to  that  of  the 
Sun.  I  present  all  these  conclusions  to  the  reader 
as  they  are  furnished  by  various  physicists  who 
have  investigated  such  matters,  but  it  is  rather 
uncertain  as  to  how  much  reliance  can  safely  be 
placed  on  such  calculations  in  detail. 

To  an  amateur  possessed  of  a  small  telescope, 
the  Sun  offers  (when  the  weather  is  above  the 
English  average  of  recent  years)  a  very  great 
and  constant  variety  of  matters  for  studious  scru- 
tiny in  its  so-called  "  spots."  To  the  naked  eye, 
or  even  on  a  hasty  telescopic  glance,  the  Sun 
presents  the  appearance  of  a  uniform  disc  of  yel- 
lowish white  colour,  though  often  a  little  atten- 
tion will  soon  result  in  the  discovery  of  a  few,  or 
it  may  be  many,  little  black,  or  blackish  patches, 
scattered  here  and  there  over  the  disc  seemingly 
without  order  or  method.  We  shall  presently 
find  out,  however,  that  this  last-named  suggestion 
is  wholly  inaccurate.  Though  commonly  called 
"  spots,"  these  dark  appearances  are  not  simple 
spots,  as  the  word  might  imply,  for  around  the 


22  THE  STORY  OF  THE  SOLAR  SYSTEM. 

rather  black  patch  which  constitutes  generally 
the  main  feature  of  the  spot  there  is  almost 

invariably  a 
fringe  of  paler 
tint  ;  whilst 
within  the 
confines  of  the 
black  patch 
which  first 
catches  the 
eye  there  is 
often  a  nucle- 
us or  inner 
portion  of  far 
more  intense 
depth  of 

FIG.  5.— Ordinary  Sun-spot,  June  22,  1885.      shade.  The  in- 
nermost    and 

darkest  portion  being  termed  the  nucleus,  the  or- 
dinary black  portion  is  known  as  umbra,  whilst 
the  encompassing  fringe  is  the  penumbra.  It 
is  not  always  the  case  that  each  individual  um- 
bra has  a  penumbra  all  to  itself,  for  several  spots 
are  occasionally  included  in  one  common  pe- 
numbra. And  it  may  further  be  remarked  that 
cases  of  an  umbra  without  a  penumbra  and  the 
contrary  are  on  record,  though  these  may  be 
termed  exceptional,  often  having  relation  to  ma- 
terial organic  changes  either  just  commencing  or 
just  coming  to  a  conclusion.  A  marked  contrast 
subsists  in  all  cases  between  the  luminosity  of  the 
penumbra  and  that  of  the  general  surface  of  the 
Sun  contiguous.  Towards  its  exterior  edge  the 
penumbra  is  usually  darker  than  at  its  inner  edge, 
where  it  comes  in  contact  with  the  umbra.  The 
outline  of  the  penumbra  is  usually  very  irregular, 


THE  SUN.  23 

but  the  umbra,  especially  in  the  larger  spots,  is 
often  of  regular  form  (comparatively  speaking  of 
course)  and  the  nucleus  (or  nuclei)  of  the  umbra 
still  more  noticeably  partakes  of  a  compactness 
of  outline. 

Spots  are  for  the  most  part  confined  to  a  zone 
extending  35°  or  so  on  each  side  of  the  solar 
equator ;  and  they  are  neither  permanent  in  their 
form  nor  stationary  in  their  position.  In  their 
want  of  permanence,  they  are  subject,  apparently, 
to  no  definite  laws,  for  they  frequently  appear 
and  disappear  with  great  suddenness. 

Their  motions  are  evidently  of  a  two-fold  na- 
ture ;  the  Sun  itself  rotates  on  its  axis,  and  the 
spots  collectively  participate  in  this  movement  of 
rotation ;  but  over  and  above  this  it  has  been 
conclusively  proved  that  sometimes  a  spot  has  a 
proper  motion  of  translation  of  its  own  independ- 
ently of  the  motion  which  it  has  in  consequence 
of  the  Sun's  axial  rotation.  Curiously  enough, 
spots  are  very  rare  immediately  under  the  Sun's 
equator.  It  is  in  the  zone  extending  from  8°  to 
20°  North  or  South,  as  the  case  may  be,  that  they 
are  most  abundant ;  or,  to  be  more  precise  still, 
their  favourite  latitude  seems  to  be  17°  or  18°. 
They  are  often  more  numerous  and  of  a  greater 
general  size  in  the  northern  hemisphere,  to  which 
it  may  be  added  that  the  zone  between  11°  and 
15°  North  is  particularly  noted  for  large  and 
enduring  spots.  A  gregarious  tendency  is  often 
very  obvious,  and  where  the  groups  are  very 
straggling  an  imaginary  line  joining  the  extreme 
ends  of  the  group  will  generally  be  found  more 
or  less  parallel  to  the  solar  equator  ;  and  not  only 
so,  but  extending  a  long  way,  or  sometimes  almost 
entirely,  across  the  whole  of  the  visible  disg, 


24  THE  STORY  OF  THE   SOLAR  SYSTEM. 

With  respect  to  the  foregoing  matters  Sir  John 
Herschel  remarked  : — u  These  circumstances  .  .  . 
point  evidently  to  physical  peculiarities  in  certain 
parts  of  the  Sun's  body  more  favourable  than  in 
others  to  the  production  of  the  spots,  on  the  one 
hand ;  and  on  the  other?  to  a  general  influence  of 
its  rotation  on  its  axis  aL  a  determining  cause  in 
their  distribution  and  arrangement,  and  would 
appear  indicative  of  a  system  of  movements  in 
the  fluids  which  constitute  its  luminous  surface; 
bearing  no  remote  analogy  to  our  trade-winds — 
from  whatever  cause  arising."  More  often  than 
not  when  a  main  spot  has  a  train  of  minor  spots 
as  followers  that  train  will  be  found  extending 
eastwards  from  the  east  side  of  the  spot,  rather 
than  in  any  other  direction. 

Spots  remain  visible  for  very  diverse  lengths 
of  time,  from  the  extreme  of  a  few  minutes  up  to 
a  few  months;  but  a  few  days  up  to,  say,  one 
month,  may,  in  a  general  way,  be  suggested  as 
their  ordinary  limits  of  endurance.  As  the  Sun 
rotates  on  its  axis  in  25}  days,  and  as  the  spots 
may  be  said  to  be,  practically  speaking,  fixed  or 
nearly  so  with  respect  to  the  Sun's  body,  no  spot 
can  remain  continuously  visible  for  more  than 
about  \2\  days,  being  half  the  duration  of  the 
Sun's  axial  rotation. 

With  regard  to  their  size,  spots  vary  as  much 
as  they  do  in  their  duration.  The  majority  of 
them  are  telescopic,  that  is,  are-  only  visible  with 
the  aid  of  a  telescope;  but  instances  are  not  un- 
common of  spots  sufficiently  large  to  be  visible 
to  the  naked  eye.  The  ancients  knew  nothing 
about  the  physical  constitution  of  the  Sun,  and 
their  few  allusions  to  the  subject  were  mere 
guesses  of  the  wildest  character.  They  were, 


THE  SUN.  25 

however,  able  to  notice  now  and  then  that  when 
the  Sun  was  near  the  horizon  certain  black  spots 
could  sometimes  be  distinguished  with  the  naked 
eye,  but  they  took  these  for  planets  in  conjunction 
with  the  Sun,  or  phenomena  of  unknown  origin. 
Earliest  in  point  of  date  of  those  who  have  left 
on  record  accounts  of  naked  eye  sun-spots  are  un- 
doubtedly the  Chinese.  In  a  species  of  Cyclo- 
paedia ascribed  to  a  certain  Ma-touan-lin  (whose 
records  of  comets  have  been  of  the  greatest  pos- 
sible use  to  astronomers),  we  find  an  account  of 
45  sun-spots  seen  during  a  period  of  904  years, 
from  301  A.  D.  to  1205  A.  D.  In  order  to  convey 
an  idea  of  the  relative  size  of  the  spots,  the  ob- 
servers compared  them  to  eggs,  dates,  plums,  etc., 
as  the  case  might  be.  The  observations  often 
extended  over  several  days;  some  indeed  to  as 
many  as  ten  consecutive  days,  and  there  seem  no 
grounds  for  doubting  the  authenticity  of  the  ob- 
servations thus  handed  down  to  us.  A  few  stray 
observations  of  sun-spots  were  recorded  in  Eu- 
rope before  the  invention  of  the  telescope.  Adel- 
mus,  a  Benedictine  monk,  makes  mention  of  a 
black  spot  on  the  Sun  on  March  17,  807.  It  is 
also  stated  that  such  a  spot  was  seen  by  Aver- 
roes  in  1161.  Kepler  himself  seems  to  have  un- 
consciously once  seen  a  spot  on  the  Sun  with  the 
naked  eye,  though  he  supposed  he  was  looking  at 
a  transit  of  the  planet  Mercury.  None  of  these 
early  observers  have  told  us  the  way  in  which 
they  made  their  observations,  but  the  smallest  of 
boys  who  has  any  claim  to  scientific  knowledge  is 
aware  of  the  fact,  that  by  the  use  of  so  simple  an 
expedient  as  a  piece  of  glass  blackened  with 
smoke,  spots  which  are  of  sufficient  size  can  be 
seen  with  the  naked  eye.  Before  telescopes  came 


26  THE   STORY   OF  THE   SOLAR   SYSTEM. 

into  use  it  was  customary  to  receive  the  solar 
rays  in  a  dark  chamber  through  a  little  circular 
hole  cut  in  a  shutter.  It  was  thus  that  J.  Fabri- 
cius  succeeded  in  December  1610  in  seeing  a  con- 
siderable spot  and  following  its  movement  suffi- 
ciently well  to  enable  him  to  determine  roughly 
the  period  of  the  Sun's  rotation. 

The  spots  may  often  be  easily  observed  with 
telescopes  of  small  dimensions,  taking  care,  how- 
ever, to  place  in  front  of  the  eye-piece  a  piece  of 
strongly-coloured  glass.  For  this  purpose  glasses 
of  various  colours  are  used,  but  none  so  good  as 
dark  green  or  dark  neutral  tint.  It  is  not  alto- 
gether easy  to  say  positively  how  large  a  spot 
must  be  for  it  to  be  visible  with  the  naked  eye, 
or  an  opera  glass,  but  probably  it  may  be  taken 
generally  that  no  spot  of  lesser  diameter  than  i' 
of  arc  can  be  so  seen.  This  measurement  must 
be  deemed  to  apply  to  that  central  portion  of  a 
normal  spot  already  mentioned  as  being  what  is 
called  the  nucleus,  because  penumbrse  may  be 
more  than  i'  in  diameter  without  being  visible  to 
the  naked  eye,  for  the  reason  that  their  shading 
is  so  much  less  pronounced  than  the  shading  of 
umbrae.  Very  large  and  conspicuous  spots  are 
comparatively  rare,  though  during  the  years  1893 
and  1894  there  were  an  unusual  number  of  such 
spots.  It  often  happens  that  a  conspicuous  group 
is  the  result  of  the  merging  or  joining  up  of  sev- 
eral smaller  groups.  In  such  cases  a  group  may 
extend  over  an  area  on  the  Sun  3'  or  4'  of  arc  in 
length  by  2'  or  3'  in  breadth.  The  largest  spot 
on  record  seems  to  have  been  one  seen  on  Sep- 
tember 30,  1858,  the  length  of  which  in  one  direc- 
tion amounted  to  more  than  140,000  miles. 

The  observation  of  spots  on  the  Sun  by  pro- 


THE  SUN.  27 

jecting  them  on  to  a  white  paper  screen  with  the 
aid  of  a  telescope  is  a  method  so  convenient  and 
so  exact  as  to  deserve  a  detailed  description,  the 
more  so  as  it  is  so  little  used.  Let  there  be  made 
in  the  shutter  of  a  darkened  room  a  hole  so  much 
larger  than  the  diameter  of  the  telescope  to  be 
used  as  will  allow  a  certain  amount  of  play  to  the 
telescope  tube,  backwards  and  forwards,  up  and 
down,  and  from  right  to  left.  Direct  the  tele- 
scope to  the  Sun  and  draw  out  the  eye-piece  to 
such  a  distance  from  the  object-glass  as  that  the 
image  projected  on  a  white  screen  held  behind 
may  be  sharply  defined  at  its  edges.  If  there  are 
any  spots  on  the  Sun  at  the  time  they  will  then 
be  seen  clearly  exhibited  on  the  screen.  An 
image  obtained  in  this  way  is  reversed  as  com- 
pared with  the  image  seen  by  looking  at  the  Sun 
through  a  telescope  directly.  If  therefore  the 
telescope  is  armed  with  the  ordinary  astronomical 
eye-piece,  which  inverts,  then  the  projection  will 
be  direct,  that  is  to  say,  on  the  screen  the  N.  S. 
E.  and  W.  points  will  correspond  with  the  same 
terrestrial  points.  Under  such  circumstances  the 
spots  will  be  seen  to  enter  the  Sun's  disc  on  the 
E.  side  and  to  go  off  on  the  W.  side.  The  con- 
trary condition  of  things  would  arise  if  a  Galilean 
telescope  or  a  terrestrial  telescope  of  any  kind 
were  made  use  of.  These  instruments  erect  the 
image,  and  therefore  will  give  by  projection  a  re- 
versed image,  in  which  we  shall  see  the  spots 
moving  apparently  in  a  direction  contrary  to  their 
true  direction. 

If  the  reader  has  grasped  the  broad  general 
outlines  now  given  respecting  the  Sun  and  its 
spots  he  will  perhaps  be  interested  to  learn  a  few 
further  details,  but  these  must  be  presented  in  a 


28  THE   STORY  OF  THE   SOLAR   SYSTEM. 

somewhat  disjointed  fashion,  because  the  multi- 
tude of  facts  on  record  concerning  sun-spots  are 
so  great  as  to  render  a  methodical  treatment  of 
them  extremely  difficult  within  the  limits  here 
imposed  on  me.  These  matters  have  been  gone 
into  in  a  very  exhaustive  way  by  Secchi  in  his 
great  treatise  on  the  Sun,  and  in  what  follows  I 
have  made  much  use  of  his  observations. 

Let  us  look  a  little  further  into  the  laws  reg- 
ulating the  movement  of  the  spots.  If  it  is  not 
a  question  of  seeing  a  spot  spring  into  view,  but 
of  watching  one  already  in  existence,  v;e  shall,  in 
general,  see  such  a  spot  appear  on  the  Eastern 
limb  of  the  Sun  just  after  having  turned  the  cor- 
ner, so  to  speak.  The  spots  traverse  the  Sun's 
disc  in  lines  which  are  apparently  oblique  with 
reference  to  the  diurnal  movement  and  the  plane 
of  the  ecliptic,  and  after  about  13  days  they  will 
disappear  at  the  Western  limb  if  they  have  not 
done  so  before  by  reason  of  physical  changes  in 
their  condition.  It  is  not  uncommon  for  a  spot 
after  remaining  invisible  for  13  days  on  the  other 
side  of  the  Sun,  so  to  speak,  to  reappear  on  the 
Eastern  limb  and  make  a  second  passage  across 
the  Sun ;  sometimes  a  third,  and  indeed  some- 
times even  a  fourth,  passage  may  be  observed,  but 
more  generally  they  change  their  form  and  vanish 
altogether  either  before  passing  off  the  visible 
disc,  or  whilst  they  are  on  the  opposite  side  as 
viewed  from  the  Earth. 

When  several  spots  appear  simultaneously, 
they  describe  in  the  same  period  of  time  similar 
paths  which  are  sensibly  parallel  to  one  another 
although  they  may  be  in  very  different  latitudes. 
The  conclusion  from  this  is  inevitable,  that  spots 
are  not  bodies  independent  of  the  Sun,  as  satel- 


THE  SUN. 


lites  would  be,  but  that  they  are  connected  with 
the  Sun's  surface,  and  are  affected  by  its  move- 
ment of  rotation.  If  we  make  every  day  for  a 
few  days  in  succession  a  drawing  of  the  Sun's 
disc  with  any  spots  that  are  visible  duly  marked 
thereon,  we  shall  see  that  their  apparent  progress 


FIG.  6. — Change  of  Form  in  Sun-spots  owing  to  the  Sun's  rotation. 

is  rapid  near  the  centre  of  the  Sun,  but  slow  near 
either  limb.  These  differences,  however,  are  ap- 
parent and  not  real,  for  their  movement  appears 
to  us  to  take  place  along  a  plane  surface,  whilst 
in  reality  it  takes  place  along  a  circle  parallel  to 
the  solar  equator.  The  spots  in  approaching  the 
Sun's  W.  limb,  if  they  happen  to  seem  somewhat 
circular  in  form  when  near  the  centre,  first  be- 


30  THE  STORY  OF   THE   SOLAR  SYSTEM. 

come  oval,  and  then  seem  to  contract  almost  into 
mere  lines.  These  changes  are  simple  effects  of 
perspective,  and  are  to  be  explained  in  the  same 
manner  as  the  apparent  decrease  in  the  size  of 
many  of  the  spots  is  often  explicable.  But  this 
condition  of  things  proves,  however,  that  the  spots 
belong  to  the  actual  surface  of  the  Sun,  for,  on  a 
contrary  supposition,  we  should  have  to  regard 
them  as  circular  bodies  greatly  flattened  like 
lozenges,  and  this  would  be  contrary  to  all  we 
know  of  the  forms  affected  by  the  heavenly  bodies. 
Of  course  besides  the  apparent  changes  of  form 
just  alluded  to  as  the  effect  of  perspective,  it  is 
abundantly  certain  that  solar  spots  often  undergo 
very  real  changes  of  form,  not  only  from  day  to 
day,  but  in  the  course  of  a  few  hours.  Several 
spots  will  often  become  amalgamated  into  one,  and 
it  was  ephemeral  changes  of  this  character  which 
hindered  generally  the  early  observers  from  deter- 
mining with  precision  the  duration  of  the  Sun's 
rotation. 

The  apparent  movements  of  the  spots  vary 
also  from  month  to  month  during  the  year  ac- 
cording to  the  season.  In  March  their  paths  are 
very  elongated  ellipses  with  the  convexity  towards 
the  N.,  the  longer  axis  of  the  ellipse  being  almost 
parallel  to  the  ecliptic.  After  that  epoch  the  cur- 
vature of  the  ecllipse  diminishes  gradually,  at  the 
same  time  that  the  major  axis  becomes  inclined 
to  the  ecliptic,  so  that  by  June  the  flattening  of 
the  ellipse  has  proceeded  so  far  that  the  path  has 
become  a  straight  line.  Between  June  and  Sep- 
tember the  elliptical  form  reappears  but  in  a  re- 
versed position  ;  then,  following  these  reversed 
phases,  the  ellipticity  decreases,  and  for  the  sec- 
ond time  there  is  an  epoch  of  straight  lines.  This 


THE  SUN.  31 

happens  in  December,  but  the  straight  lines  are 
inclined  in  a  converse  direction  to  that  which  was 
the  case  in  June.  It  must  again  be  impressed  on 
the  reader  that  all  these  seemingly  different  forms 
of  path  pursued  by  the  spots  are  merely  effects  of 
perspective,  for  in  reality,  the  spots  in  crossing 
the  Sun's  disc  describe  lines  which  are  virtually 
parallel  to  the  solar  equator.  These  projections 
really  depend  of  course  on  the  position  of  the 
observer  on  the  Earth,  and  vary  as  his  position 
varies  during  the  Earth's  annual  circuit  round 
the  Sun.  The  number  of  the  spots  varies  through 
wide  limits.  Sometimes  they  are  so  numerous  that 
a  single  observation  will  enable  us  to  recognise  the 
position  of  the  zones  of  maximum  frequency. 
Sometimes,  on  the  other  hand,  they  are  so  scarce, 
that  many  weeks  may  pass  away  without  hardly 
one  being  seen.  A  remarkable  regularity  is  now 
recognised  in  the  succession  of  these  periods  of 
abundance  and  scarcity,  as  we  shall  see  later  on. 

It  is  both  useful  and  interesting  in  studying 
the  spots  to  record  methodically  their  number 
and  their  size,  but  it  is  not  easy  to  teach  observ- 
ers how  to  do  this  so  systematically  that  observa- 
tions by  one  person  can  be  brought  into  compari- 
son with  those  of  another.  Photography  and 
hand-drawing  on  a  screen  alone  furnish  a  trust- 
worthy basis  of  operations.  Spots  in  general  may 
naturally  be  classified  into  (i)  isolated  spots  or 
points,  and  (2)  groups  of  spots;  but  often  one 
observer  will  describe  as  a  small  spot  an  object 
which  another  observer  would  regard  as  a  mere 
point;  and  one  observer  will  record  several 
groups  where  another  observer  will  see  but  one. 
A  very  few  days'  experience  with  a  telescope  will 
bring  home  to  the  observer's  mind  the  difficulty 
3 


32  THE   STORY  OF  THE   SOLAR  SYSTEM. 

of  dealing  with  the  spots  where  it  is  a  question  of 
systematic  methodical  observation  of  them. 

Let  us  now  take  a  brief  survey  of  some  of  the 
theories  which  have  been  put  forth  regarding  the 
nature  of  the  spots  on  the  Sun.  In  the  early  days 
of  the  telescope,  that  is  to  say,  during  the  i;th 
century,  two  general  ideas  were  current.  Some 
thought  the  spots  to  be  shapeless  satellites  re- 
volving round  the  Sun;  others  that  they  were 
clouds,  or  aggregations  of  smoke,  floating  about 
in  a  solar  atmosphere.  Scheiner,  the  author  of 
the  first  theory,  abandoned  it  towards  the  close 
of  his  life,  having  arrived  at  the  conclusion  that 
the  spots  were  situated  below  the  general  level 
of  the  Sun's  surface.  Another  idea,  but  of  later 
date,  was  that  the  Sun  is  a  liquid  and  incan- 
descent mass  of  matter,  and  the  spots  immense 
fragments  of  Scorice,  or  clinkers,  floating  upon  an 
ocean  of  fire. 

Somewhat  more  than  a  century  after  the  spots 
had  been  generally  studied  with. the  aid  of  a  tele- 
scope a  Scotchman  named  Wilson  made  a  memo- 
rable discovery.  He  showed  by  the  clearest  evi- 
dence that  they  are  cavities,  and  he  propounded 
the  first  intelligible  idea  of  the  true  physical  con- 
stitution of  the  Sun,  when  he  compared  to  a 
strongly  illuminated  cloud  the  luminous  layer  of 
solar  material  which  we  now  term  the  "  photo- 
sphere." On  November  22,  1769,  he  observed  on 
the  Sun's  disc  a  fine  round  spot  encompassed  by 
a  penumbra,  also  circular,  and  concentric  with 
the  nucleus.  He  watched  that  spot  up  to  the 
time  that  it  disappeared,  and  he  soon  remarked 
that  the  penumbra  ceased  to  be  symmetrical:  the 
part  turned  towards  the  centre  of  the  Sun  became 
smaller  and  smaller,  and  eventually  disappeared 


THE  SUN.  33 

altogether  ;  whilst  the  part  on  the  opposite  side 
preserved  its  fulness  and  dimensions  almost  un- 
changed. Let  us  suppose  we  chanced  to  turn  a 
telescope  on  to  the  Sun  on  a  given  day,  and  were 
fortunate  enough  to  discover  a  spot  in  the  centre 
of  the  disc,  with  a  penumbra  concentric  with  the 
nucleus.  When  such  a  spot  arrives  about  midway 
towards  the  limb,  it  will  exhibit  a  penumbra  nar- 
rower on  the  left  side  than  on  the  right ;  later  on 
the  penumbra  will  disappear  almost  or  quite  com- 
pletely on  the  left  side  :  then  the  nucleus  itself 
will  seem  to  be  encroached  upon.  Finally,  very 
near  the  limb,  there  will  remain  only  a  slender 
thread  of  penumbra,  and  the  nucleus  will  have 
ceased  to  be  directly  visible.  Such  were  the 
phases  of  transformation  observed  by  Wilson  and 
often  studied  since.  Wilson  s-uspected  that  he 
had  come  upon  some  great  law  that  was  ripe  for 
disclosure,  and  in  order  not  to  be  misled  he 
waited  for  the  return  of  the  same  spot,  which  in- 
deed reappeared  on  the  Sun's  W.  limb  after 
about  14  days.  Then  he  found  himself  face  to 
face  with  the  same  phases  reproduced,  but  in  the 
reverse  order :  the  penumbra  contracted  on  one 
side  and  full  on  the  other,  widening  out  on  the 
contracted  side  as  the  spot  came  up  to  the  Sun's 
centre.  Henceforth  doubt  was  no  longer  possi- 
ble ;  the  spot  had  sensibly  preserved  the  same 
shape  during  its  passage,  and  the  alterations 
noticed  were  only  apparent,  and  resulted  from  an 
effect  of  perspective  which  was  easy  to  be  under- 
stood. The  different  phases  presented  by  such  a 
spot  as  that  just  spoken  of  will  be  so  much  the 
more  sensible  according  as  the  depth  of  the  cav- 
ity is  greater  ;  but  if  the  depth  is  inconsiderable 
the  bottom  of  the  cavity  will  only  disappear  when 


34  THE  STORY   OF  THE   SOLAR  SYSTEM. 

a  very  oblique  angle  is  attained,  and  this  cannot 
happen  except  when  the  spot  is  very  near  to  the 
limb.  By  observations  carefully  made  under  such 
circumstances  it  will  be  possible  to  determine  the 
depth  of  the  cavity,  and  Wilson  found  that  the 
depth  of  a  spot  often  amounted  to  about  one- 
third  of  the  Earth's  radius.  Wilson's  theory  was 
not  accepted  without  dispute;  it  was  contested 
by  several  astronomers,  and  in  particular  by 
Lalande.  It  was  however  taken  up  by  Sir  W. 
Herschel,  and  as  modified  by  him  has  met  with 
general  acceptance  down  to  the  present  time ; 
though  now  and  again  challenged,  perhaps  most 
recently  and  most  vehemently  by  Hewlett,  a  sun 
spot  observer  of  great  experience.  Wilson's  dis- 
covery was  the  point  of  departure  for  the  grand 
labours  of  Sir  W.  Herschel  in  the  field  of  Solar 
Physics.  Man  of  genius  that  Herschel  was,  he 
was  above  all  things  an  observer  who  took  his 
own  line  in  what  he  did.  He  saw  so  many  phe- 
nomena with  the  powerful  instruments  con- 
structed by  himself,  he  described  so  minutely 
the  marvels  which  were  revealed  to  him,  that  he 
left  comparatively  little  for  his  successors  to  do 
so  far  as  regards  mere  telescopic  observation. 
Herschel's  main  idea  as  to  the  Sun  was  based  on 
Wilson's  discovery.  He  remarked  with  reason, 
as  that  astronomer  had  done,  that  if  the  spots 
are  cavities  the  luminous  matter  could  neither  be 
properly  called  liquid  nor  gaseous;  for  then  it 
would  precipitate  itself  with  frightful  rapidity  to 
fill  up  the  void,  and  that  would  render  it  impos- 
sible that  the  spots  should  endure  as  we  often  see 
they  do  during  several  revolutions  of  the  Sun. 
Moreover,  the  proper  movements  of  the  spots 
prove  that  the  photosphere  is  not  solid.  We  can 


THE  SUN.  35 

therefore  only  liken  it  to  fogs  or  clouds,  and  it 
must  be  suspended  in  an  atmosphere  similar  to 
ours.  Such  is,  according  to  Herschel,  the  only 
hypothesis  which  can  explain  the  rapid  changes 
which  we  witness.  We  shall  see  a  little  later  on 
that  these  phenomena  do  admit  of  another  expla- 
nation. 

In  a  second  memoir  Herschel  followed  up  this 
inquiry  with  an  acuteness  worthy  of  his  genius. 
Unfortunately  he  allowed  himself  to  be  carried 
away  with  the  idea  that  the  Sun  was  inhabited  in 
order  to  sustain  this  theory.  He  needed  a  solid 
kernel  upon  which  his  imaginary  inhabitants  could 
dwell ;  and  also  a  means  whereby  he  could  protect 
them  from  the  radiations  of  the  photosphere. 
With  this  idea  in  view  he  conjectured  the  exist- 
ence above  the  Sun's  solid  body  of  a  layer  of 
clouds  always  contiguous  to  the  photosphere 
which  enveloped  it,  and  which  always  being  rent 
when  the  photosphere  was  rent,  thus  enabled  us 
to  see  the  solid  body  of  the  Sun  lying  behind. 
These  notions  can  only  be  described  as  very  ar- 
bitrary, as  unsupported  by  observation,  and  as 
involving  explanations  quite  out  of  harmony  with 
the  principles  of  modern  physics.  However,  the 
labours  of  Herschel  resulted  in  so  many  positive 
discoveries  of  visible  facts,  and  in  so  many  just 
conclusions,  that  they  contributed  greatly  to  the 
growth  of  our  present  knowledge  of  the  true  con- 
stitution of  the  Sun. 

Since  Wilson's  time,  as  Secchi  pointedly  re- 
marks, astronomers  generally  have  verified  his 
observations  with  good  instruments,  and  by  an 
investigation  of  a  great  number  of  spots.  De  La 
Rue,  discussing  the  Kew  observations,  found  that 
of  89  regular  spots  72  gave  results  which  con- 


36  THE   STORY  OF  THE   SOLAR   SYSTEM. 

formed  to  Wilson's  ideas,  whilst  the  remaining  17 
were  opposed  thereto.  There  is  nothing  sur- 
prising in  the  existence  of  a  contrarient  minority 
when  we  consider  the  great  changes  which  in 
reality  often  occur  in  the  forms  of  the  spots.  De 
La  Rue  suggested  a  very  simple  expedient  for 
showing  that  the  spots  are  cavities.  Take  two 
photographs  of  the  Sun  made  at  an  interval  of 
one  day :  during  that  time  every  point  on  the 
Sun's  surface  will  have  been  displaced,  so  far  as 
the  telescope  is  concerned,  by  about  15°.  Place 
these  photographs  in  a  stereoscope,  and  we  shall 
readily  see  the  interior  cavity,  the  edges  of  which 
will  appear  raised  above  the  photosphere.  It  is 
impossible  therefore  to  entertain  the  least  doubt 
as  to  the  truth  of  the  theory  that  the  spots  are 
excavations  in  the  luminous  stratum  which  en- 
velopes the  whole  of  the  solar  globe. 

If  it  be  true  that  a  spot  is  a  cavity,  it  follows 
that  when  it  reaches  the  margin  of  the  solar  disc 
we  ought  to  detect  a  hollow  place  ;  and  this  will 
be  so  much  the  more  easy  to  observe  according  as 
the  cavity  is  larger  and  deeper.  As  a  matter  of 
fact,  numerous  observations  of  this  sort  have  been 
recorded  from  the  time  of  Cassini  down  to  the 
present  time  under  the  designation  of  "  notches  " 
on  the  Sun's  limb.  On  July  8,  1873,  Secchi  ob- 
served such  a  notch  8",  or  3600  miles  deep. 

Faye  and  some  other  astronomers  are  disposed 
to  support  a  theory  according  to  which  the  spots 
are  nothing  else  than  aerial  cyclones,  but  this 
does  not  seem  admissible.  If  the  fundamental 
principle  of  a  spot  is  that  it  arises  from  a  whirl- 
ing movement,  the  rays  (so  to  speak)  which  com- 
pose the  penumbrae  must  always  be  crooked,  or 
the  theory  falls  to  the  ground.  It  is  quite  true 


THE  SUN. 


37 


that  indications  of  cyclonic  action  do  sometimes 
appear,  but  they  are  at  any  rate  very  rare,  for 
only  a  small  percentage  exhibit  in  a  distinct  man- 
ner a  spiral  structure.  Moreover,  when  such  a 
structure  is  seen  it  does  not  endure  for  the  whole 
lifetime  of  the  spot  but  only  for  a  day  or  two  :  the 
spot  may  last  a  long  time  after  it  has  lost  its  spiral 
features,  if  it  ever  had  any.  Sometimes  even  the 


FIG.  7.— Sun-spot  seen  as  a  Notch. 

whirling  movement,  after  having  slackened,  begins 
again,  but  in  the  contrary  direction.  Under  these 
circumstances,  though  this  occasional  spiral  struc- 
ture is  very  curious  and  interesting,  we  are  not 
justified  in  taking  it  as  the  basis  of  a  theory  which 
has  any  pretensions  to  explain  the  general  nature 
of  sun-spots. 


38  THE   STORY   OF  THE  SOLAR  SYSTEM. 

When  we  examine  the  Sun  with  instruments  of 
large  aperture  and  high  magnifying  power,  we  no- 
tice that  its  surface  is  far  from  being  as  smooth 
and  uniform  as  it  appears  in  a  small  telescope. 
On  the  contrary,  it  presents  an  irregular  undulat- 
ing appearance  like  a  pond  or  other  sheet  of  water 
agitated  by  the  wind.  Careful  scrutiny  with  a 
powerful  eye-piece  reveals  the  fact  that  the  Sun's 
surface  is  marked  by  a  multitude  of  wrinkles  and 
irregularities  which  it  is  well-nigh  impossible  to 
describe  in  words.  More  or  less  everywhere  there 
is  a  general  mottling  visible  ;  it  is  more  distinct 
in  some  places  than  others,  and  especially  so 
towards  the  centre  of  the  disc.  This  peculiar 
appearance  varies  very  much  from  time  to  time, 
and  its  distinctness  seems  to  depend  a  great  deal 
on  the  state  of  the  Earth's  atmosphere,  for  it  be- 
comes invisible  when  the  air  is  disturbed  ;  but 
these  variations  depend  also  on  real  variations  of 
the  photosphere — a  fact  which  observations  made 
in  very  calm  weather  are  thought  clearly  to  indi- 
cate. 

It  is  often  said  that  the  Sun  exhibits  a  granu- 
lated structure.  If  we  wish  to  realise  in  the  most 
precise  manner  what  is  meant  by  the  word  "gran- 
ulation "  as  applied  to  the  structure  of  the  Sun, 
we  must  abandon  the  method  of  projection  and 
examine  the  Sun  directly  with  a  powerful  eye- 
piece, taking  advantage  of  a  moment  when  the 
atmosphere  is  perfectly  calm,  and  before  the  eye- 
piece has  had  time  to  get  hot.  It  may  then  be 
seen  that  the  Sun's  surface  is  covered  with  a  mul- 
titude of  little  grains,  nearly  all  of  about  the  same 
size,  but  of  different  shape,  though  for  the  most 
part  more  or  less  oval.  The  small  interstices 
which  separate  these  grains  form  a  net-work 


THE  SUN.  39 

which  is  dark  without  being  positively  black. 
Secchi  considered  it  difficult  to  name  any  known 
object  which  exactly  answers  in  appearance  to 
this  structure,  but  he  thought  that  we  can  find 
something  resembling  it  in  examining  with  a  mi- 
croscope milk  which  has  been  a  little  dried  up,  and 
the  globules  of  which  have  lost  their  regular  form. 
Exceptionally  good  atmospheric  conditions  are  un- 
der all  circumstances  indispensable  for  the  study 
of  these  details. 

In  point  of  fact,  there  is  a  mysterious  uncer- 
tainty about  the  normal  condition  of  the  Sun's 
surface,  in  a  visual  sense,  which  a  few  years  ago 
engendered  a  very  vehement  controversy,  and  led 
to  the  use  of  such  expressions  as  "  willow  leaves," 
"  rice  grains,"  "  sea  beach,"  and  "  straw  thatching," 
to  indicate  what  was  seen.  All  these  words  are 
too  precise  to  be  quite  suitable  to  be  taken  lit- 
erally, but  perhaps,  on  the  whole,  "  rice  grains  " 
is  not  altogether  a  bad  expression  to  recall  what 
certainly  seems  to  be  the  granular  surface  of  the 
Sun  as  we  see  it. 

By  making  use  of  moderate  magnifying  powers, 
what  we  see  will  often  convey  the  impression  of 
a  multitude  of  white  points  on  a  black  net-work. 
This  is  very  apparent  during  the  first  few  mo- 
ments that  the  telescope  is  brought  to  bear  on 
the  Sun,  but  its  clearness  quickly  passes  away 
because  the  eye  gets  fatigued,  and  the  lenses 
becoming  warm  the  air  in  the  telescope  tube  gets 
disturbed  because  also  warmed.  Sometimes  the 
appearance  is  a  little  different  from  that  just 
described,  and  along  with  the  white  and  brilliant 
points  little  black  holes  are  intermixed.  Often- 
times the  grains  appear  as  if  suspended  in  a  black 
net-work  and  heaped  together  in  knots  more  or 


40  THE  STORY  OF  THE  SOLAR   SYSTEM. 

less  shaded  and  more  or  less  broad.  Sometimes 
the  grains  exhibit  a  very  elongated  form,  especially 
in  the  neighbourhood  of  the  spots.  It  is  these 
elongated  forms  to  which  Nasmyth  applied  the 
term  "  willow  leaf,"  whilst  Huggins  thought 
"  rice  grains  "  a  very  suitable  expression. 

This  granular  or  leaf- like  structure — call  it 
what  we  will — cannot  be  made  out  except  with 
considerable  optical  assistance,  for  the  grains 
being  intrinsically  very  small,  diffraction  in  en- 
larging them  and  causing  them  to  encroach  on 
one  another  necessarily  produces  a  general  con- 
fusion of  image.  The  real  dimensions  of  these 
grains  cannot  therefore  readily  be  determined 
by  direct  measurement,  but  by  comparing  them 
with  the  wires  used  in  micrometer  eye-pieces  it 
has  been  thought  that  their  diameters  may  usu- 
ally be  regarded  as  equal  to  %  or  -J  of  a  second — 
say  from  120  to  150  miles.  The  granules  seem  to 
be  possessed  of  sensible  movement,  but  presumably 
it  is  not  always  or  even  generally  a  movement  of 
translation  from  place  to  place  ;  only  an  undulatory 
movement  like  that  of  still  water  when  a  stone  is 
cast  into  it.  Nevertheless,  probably  in  certain 
cases  the  granules  actually  are  affected  by  a 
motion  of  translation,  for  in  the  vicinity  of  spots 
they  may  sometimes  be  seen  flowing  over  the 
edges  of  the  penumbrae.  In  order  to  explain  the 
existence  of  the  granules  the  strangest  theories 
have  been  broached.  Sir  William  Herschel  hav- 
ing observed  the  granulations,  applied  to  them 
the  term  **  corrugations  "  or  "  furrows  " — words 
somewhat  inexact,  perhaps,  but  by  which,  as  his 
descriptions  clearly  show,  he  meant  to  designate 
the  features  which  I  am  now  treating  of.  He 
even  noticed  the  dark  network  which  separates 


THESUN-  41 

the  grains,  and  he  applied  to  it  the  word  "  inden- 
tations." 

These  granulations  are  without  doubt  promi- 
nences, probably  of  hydrogen  gas,  which  rise 
above  the  general  surface,  for  this  structure  is 
much  more  sharp  and  distinct  at  the  centre  of 
the  sun's  disc  than  at  the  limbs  ;  that  is  to  say, 
near  the  limbs  of  the  Sun  they  partially  overlap 
one  another,  as  indeed  Herschel  remarked.  The 
idea  of  flames  would  satisfy  these  appearances : 
and  as  the  spectroscope  suggests  to  us  that  the 
Sun  is  habitually  covered  over  with  a  multitude 
of  little  jets  of  flame,  the  observations  which  have 
been  made  compel  the  opinion  that  the  grains  are 
the  summits  of  those  prominences  which  exist  all 
over  the  Sun's  surface. 

The  surface  is  sometimes  so  thickly  covered 
over  with  these  granulations — the  network  is  so 
conspicuous — that  we  can  readily  imagine  that 
we  see  everywhere  pores  and  the  beginnings  of 
spots,  but  this  aspect  is  not  permanent,  and  seems 
to  depend  to  some  extent  on  atmospheric  causes 
combined  also  with  actual  changes  in  the  Sun's 
surface  itself.  There  seems  however  no  doubt 
that  the  joints,  so  to  speak,  of  the  dark  network 
already  referred  to  do  sometimes  burst  asunder 
and  develope  into  spots. 

The  circumstances  which  accompany  the  for- 
mation of  a  spot  cannot  readily  be  specified  with 
certainty.  It  is  impossible  to  say  that  there 
exists  any  law  as  to  this  matter.  Whilst  some 
spots  develope  very  slowly  by  the  expansion  of 
certain  pores,  others  spring  into  existence  quite 
suddenly.  Yet  it  cannot  be  said  that  the  forma- 
tion of  a  spot  is  ever  completely  instantaneous 
however  rapid  it  may  be.  The  phenomenon  is 


42  THE  STORY  OF  THE   SOLAR   SYSTEM. 

often  announced  some  days  in  advance  :  we  may 
perceive  in  the  photosphere  a  great  agitation 
which  often  manifests  itself  by  some  very  brilliant 
faculce.  (to  be  described  presently)  giving  birth  to 
one  or  more  pores.  Very  often  we  next  notice 
some  groups  of  little  black  spots,  as  if  the  lu- 
minous stratum  was  becoming  thinner  in  such  a 
way  as  to  disappear  little  by  little  and  leave  a 
large  black  nucleus  uncovered.  At  the  com- 
mencement of  the  business  there  is  usually  no 
clearly  denned  penumbra.  This  developes  itself 
gradually  and  acquires  a  regular  outline,  just  as 
the  spot  itself  often  takes  a  somewhat  circular 
form.  This  tranquil  and  peaceable  formation  of 
a  spot  only  happens  at  a  time  when  calm  seems 
to  reign  in  the  solar  atmosphere :  in  general  the 
development  is  more  tumultuous  and  the  stages 
more  complicated. 

As  a  rule  a  spot  passes  through  three  stages  of 
existence  : — (i)  the  Period  of  birth  ;  (2)  a  Period 
of  calm  ;  arid  (3)  the  Period  of  dissolution.  When 
a  spot  is  on  the  point  of  closing  up,  the  flow  of 
the  luminous  matter  which  it,  as  it  were,  attracts, 
is  not  directed  uniformly  towards  the  centre;  it 
seems  that  the  photospheric  masses,  no  longer 
meeting  with  resistance,  are  precipitated  promis- 
cuously anywhere  so  as  to  fill  up  the  hole.  It  is 
impossible  to  describe  in  detail  the  phases  which 
irregular  spots  go  through,  but  two  things  may 
always  be  remarked :  that  their  structure  is  char- 
acterized by  the  existence  of  luminous  filaments, 
and  that  these  filaments  converge  towards  one  or 
several  centres. 

Secchi  thus  sums  up  certain  conclusions  which 
he  arrived  at  relating  to  spots  generally: — (i)  It 
is  not  on  the  surface  of  any  solid  body  that  the 


THE  SUN.  43 

solar  spots  are  manifested  ;  they  are  produced  in 
a  fluid  mass,  the  fluidity  of  which  is  represented 
by  a  gas,  so  that  the  constitution  of  this  medium 
may  be  likened  to  that  of  flames  or  clouds;  (2) 
the  known  details  respecting  the  constitution  of 
the  penumbra  and  the  phenomena  exhibited  prove 
that  the  penumbra  is  not  a  mass  of  obscure  mat- 
ter which  floats  across  luminous  matter,  but  that 
it  is  on  the  contrary  a  case  of  luminous  matter 
invading  and  floating  about -over  darker  materials 
and  so  producing  a  half  tint. 

All  the  available  evidence  which  we  possess 
may  be  said  to  show  that  the  spots  are  not  merely 
superficial  appearances,  but  that  they  have  their 
origin  deep  in  the  interior  of  the  Sun,  and  are 
produced  by  the  operation  of  causes  still  unknown 
to  us  which  affect  and  disturb  the  Sun's  mass  to 
an  extent  which  is  sometimes  very  considerable. 
The  spots  then  are  only  the  results  of  a  great 
agitation  in  the  materials  of  which  the  Sun  is 
composed,  and  this  agitation  extends  far  down 
below  the  limits  of  the  visible  dark  nucleus  what- 
ever that  may  consist  of. 

Besides  the  spots,  streaks  of  light  may  fre- 
quently be  remarked  upon  the  surface  of  the  Sun 
towards  the  margin  of  the  disc.  These  are  termed 
facultz  (torches),  and  they  are  often  found  near 
the  spots,  or  where  spots  have  previously  existed 
or  have  afterwards  appeared.  When  quite  near 
the  Sun's  limb  these  faculae  are  usually  more  or 
less  parallel  to  the  limb.  'They  are  of  irregular 
form  and  may  be  likened  to  certain  kinds  of  coral. 
They  generally  appear  to  be  more  luminous  than 
the  solar  surface  immediately  adjacent  to  them, 
but  it  is  not  improbable  that  this  is  an  optical  illu- 
sion depending  upon  the  fact  that  the  edges  of 


44  THE  STORY   OF  THE  SOLAR  SYSTEM. 

the  Sun  always  appear  much  more  luminous  than 
the  centre.  This  last-named  fact  may  be  readily 
recognised  by  the  employment  of  a  high  magnify- 
ing power,  and  moving  the  telescope  rapidly  from 
the  limb  to  the  centre  of  the  disc.  If  the  Sun  be 
projected  on  a  screen,  as  already  mentioned,  this 
degradation  of  the  Sun's  light  from  centre,  to  cir- 
cumference becomes  particularly  manifest. 

After  having  studied  the  structure  and  the 
movement  of  the  spots,  one  is  naturally  led  to  ask 
if  their  apparitions  at  different  periods  are  sub- 
ject to  any  general  law.  This  question  is  one 
which  has  much  engaged  the  attention  of  modern 
astronomers.  The  older  observers  noticed  that 
the  number  of  the  spots  visible  differed  in  differ- 
ent years.  There  were  said  to  have  been  periods 
when  months  and  even  years  passed  away  without 
any  spots  being  observed.  Even  allowing  that 
this  statement,  so  far  as  "years"  are  concerned, 
might  be  exaggerated,  and  that  the  absence  of 
spots  was  due  to  the  want  of  sufficient  care  in 
making  the  observations,  and  especially  to  the 
want  of  efficient  instruments,  it  is  none  the  less 
true  that  the  number  of  the  spots  is  extremely 
variable,  and  that  there  have  been  epochs  when 
they  were  very  scarce. 

Sir  W.  Herschel  was  the  first  who  devoted 
himself  to  the  question  of  seeking  to  establish  a 
relation  between  the  variation  of  the  spots  and 
terrestrial  meteorology.  -  For  the  want  of  any 
better  object,  he  compared  the  annual  number  of 
the  spots  with  the  price  of  wheat ;  but  it  is  easy 
to  see  that  nothing  could  result  from  such  a 
comparison.  Without  doubt  the  meteorological 
phenomena  of  the  globe  must  depend  to  some  ex- 
tent on  solar  changes:  but  the  term  of  compari- 


THE  SUN.  45 

son  selected  by  Herschel  had  no  direct  bearing 
on  the  state  of  the  Sun. 

In  our  time  this  question  has  been  investi- 
gated to  its  very  foundation  by  Wolf,  Director  for 
many  years  at  the  Observatory  of  Zurich.  It  is 
to  his  zeal  that  we  owe  a  very  interesting  assem- 
blage of  old  observations  which  were  buried  in 
archives  and  chronicles.  It  was  he  who  endeav- 
oured to  reduce  them  into  a  systematic  form,  so 
as  to  supply  as  far  as  possible  the  numerous  gaps 
which  exist  in  the  different  series. 

The  two  most  attentive  observers  at  the  period 
when  the  spots  were  discovered  were  Marriott  at 
Oxford  and  Scheiner  at  Ingoldstadt,  but  Scheiner 
himself  has  informed  us  that  he  did  not  note  down 
all  the  spots  which  he  saw;  he  only  recorded 
those  which  were  likely  to  assist  him  in  his  spe- 
cial task  of  determining  the  period  of  the  Sun's 
rotation.  Several  observers  after  him  made  iso- 
lated series  of  observations;  but  some  of  these 
have  been  lost  and  the  others  show  important 
gaps.  J.  G.  Staudacher,  at  Nuremburg,  observed 
the  Sun  with  great  perseverance  during  fifty  years 
from  1749  to  1799.  Before  him  the  Cassinis,  Mar- 
aldi,  and  others  were  engaged  in  the  same  sort  of 
work,  but  only  in  an  indirect  way  :  that  is  to  say, 
they  contented  themselves,  whilst  making  merid- 
ional observations  of  the  Sun,  with  noting  any- 
thing in  the  way  of  spots  which  they  deemed 
important.  Zucconi  and  Flaugergues  also  left 
behind  them  a  good  collection  of  observations 
which  Wolf  utilised,  rendering  them  comparable 
one  with  another  by  applying  suitable  corrections. 
The  great  difficulty  herein  arises  from  the  fact 
that  the  observers  were  not  provided  with  instru- 
ments of  equal  power ;  one  man,  armed  with  a 


46  THE  STORY  OF  THE  SOLAR  SYSTEM. 

better  telescope  than  his  contemporaries,  natu- 
rally observed  and  recorded  spots  which  would 
escape  the  others.  The  numbers  entered  in  their 
registers  are  therefore  not  comparable  inter  se. 
Wolf  endeavoured  to  replace  these  numbers  by 
others  which  would  represent  the  spots  which 
might  have  been  seen  if  the  observers  had  all 
employed  telescopes  of  a  given  kind  and  power. 
The  result  of  his  efforts  in  this  direction  is  an  al- 
most continuous  series  of  Sun-spot  records  from 
an  epoch  sufficiently  remote,  up  to  the  time  when 
this  branch  of  science  was  taken  up  with  the 
vigour  of  modern  scientific  methods. 

The  observer  who  most  assiduously  devoted 
himself  to  this  subject  in  modern  times  was 
Schwabe  of  Dessau.  From  1826  to  1868  he  never 
failed  to  make  daily  observations  when  the 
weather  permitted  him.  His  series  of  records  is 
specially  valuable,  for  Carrington's  fits  in  with  it, 
and  with  that  in  turn  Sporer's  is  comparable,  and 
the  chain  is  complete  by  the  later  photographic 
and  other  observations.  All  these  Sun-spot  rec- 
ords, though  differing  in  their  details,  may  easily 
be  used  together  when  it  is  a  question  of  working 
out  relative  annual  fluctuations. 

At  the  present  time  there  are  many  Astrono- 
mers who  are  engaged  in  observing  the  spots 
with  care ;  but  just  as  formerly  there  are  few 
who  possess  sufficient  perseverance.  The  photo- 
graphic method  is  excellent,  but  it  takes  much 
time  and  is  costly.  Some  have  decried,  in  a  very 
unreasonable  manner,  a  drawing  made  by  hand: 
such  a  drawing,  of  sufficient  size,  and  executed 
by  projection  by  a  skilful  draughtsman  with  a 
telescope  driven  by  clockwork,  may  stand  com- 
parison with  a  photograph,  and  this  method  has 


THE  SUN.  47 

a  better  chance  of  being  persevered  in.  The 
Rev.  F.  Hewlett's  name  must  be  mentioned  in 
this  connection  as  a  draughtsman  who  has  ac- 
complished much  by  hand  drawing.  Though  the 
once  famous  Kew  observations  have  been  discon- 
tinued, they  have  been  replaced  by  a  new  series 
at  Greenwich  with  similar  appliances ;  whilst 
Janssen  at  Meudon  has  also  been  carrying  on  for 
a  number  of  years  a  splendid  course  of  photo- 
graphic records. 

Schwabe,  when  he  had  collected  a  consider- 
able number  of  observations,  recognised  clear  in- 
dications of  periodicity.  Very  definite  epochs  of 
maxima  and  minima  succeeded  one  another  at 
intervals  of  10  or  n  years.  It  is  true  that  in  fol- 
lowing out  such  a  study  the  observations  are  cer- 
tain to  be  in  a  sense  a  little  defective.  At  first  it 
was  not  possible  to  observe  the  Sun  every  day, 
and  the  gaps  which  resulted  from  bad  weather 
necessarily  added  to  the  number  of  days  which 
had  to  be  set  down  as  being  without  spots.  More- 
over, every  method  of  numbering  the  spots  must 
be  a  little  arbitrary  :  there  are  often  groups 
which,  in  consequence  of  their  sub-divisions,  may 
be  counted  in  different  ways :  but  in  a  mass  of 
observations  so  considerable  as  those  of  Schwabe's, 
such  uncertainties  will  compensate  for  one  another 
and  will  disappear  in  the  final  result.  In  fact  the 
law  is  so  striking  that  it  suffices  to  cast  one's  eye 
over  his  table*  to  see  that. 

That  table  is  both  interesting  and  instructive 
at  the  same  time.  The  numbers  exhibited  in  it 
speak  for  themselves,  and  it  is  sufficient  to  exam- 

*  Given  in  full  in  my  Handbook  of  Astronomy,  4th  ed.,  vol. 
i.,  p.  26. 

4 


48  THE   STORY  OF  THE  SOLAR  SYSTEM. 

ine  them  with  even  a  small  amount  of  attention 
to  realise  the  certainty  of  the  conclusions  which 
have  been  drawn. 

It  is  therefore  now  to  be  deemed  an  ascer- 
tained fact  that  there  are  periodical  maxima  and 
minima  in  the  display  of  spots,  and  that  the  ex- 
tent of  the  period  is  between  10  and  12  years.  In 
order  to  determine  this  value  with  the  utmost 
exactness,  some  astronomers  have  had  recourse 
to  early  observations.  Wolf  of  Zurich  made  this 
the  subject  of  some  very  interesting  inquiries.  He 
was  able  to  establish  the  chronology  of  the  phases 
which  the  Sun  has  passed  through  from  the  time 
of  the  first  discovery  of  the  spots  to  the  present 
day — more  than  2^  centuries.  His  calculations 
led  him  to  a  period  of  n^  years.  Lamont  fixed 
upon  10.43  years,  but  this  number  does  not  repre- 
sent the  more  recent  observations  with  sufficient 
precision. 

In  order  to  exhibit  this  law  in  the  plainest 
possible  manner  the  dates  of  maxima  and  minima 
should  be  laid  down  on  ruled  paper  in  proper 
mathematical  form,  the  abscissa  of  the  curve  rep- 
resenting the  years,  and  the  ordinates  the  number 
of  spots  observed. 

An  examination  of  a  curve  thus  plotted  shows 
two  things: — (i)  That  the  period  is  clearly  an 
eleven-year  one,  as  has  been  already  stated;  (2) 
that  it  is  not  however  quite  as  simple  in  its  form 
as  it  was  at  first  thought  to  be;  for  in  reality 
there  are  two  periods  superposed,  the  one  rather 
more  than  half  a  century  long,  and  the  other  ex- 
tending over  the  n  years  already  spoken  of. 
We  do  not  possess  early  observations  sufficiently 
numerous  and  sufficiently  good  to  enable  us  to 
draw  any  unimpeachable  conclusions  as  to  the 


THE  SUN.  49 

nature  of  the  long  period ;  we  can  only  be  certain 
that  it  exists.  The  later  labours  of  Wolf,  how- 
ever, fixed  that  period  at  55^  years.  It  is  a  re- 
sult of  this  that,  according  to  Loomis,  a  period  of 
comparative  calm  on  the  Sun  existed  between  1810 
and  1825. 

Each  maximum  lies  nearer  to  the  minimum 
which  precedes  it  than  to  the  minimum  which  fol- 
lows it,  for  the  spots  increase  during  3.7  years, 
and  then  diminish  during  7.4  years.  According 
to  De  La  Rue  the  increase  occupies  3.52  years, 
and  diminution  7.55  years.  This  concurrence  be- 
tween De  La  Rue  and  Wolf  is  surprising  consid- 
ering the  diversity  of  the  methods. which  led  to 
results  almost  identical,  the  one  set  being  based 
on  the  number  of  the  spots,  and  the  other  on  the 
superficial  extent  of  the  spots.  The  different 
periods  in  succession  are  not  absolutely  identical: 
but  it  has  been  remarked  that  if  during  any  one 
period  the  decrease  is  retarded  or  accelerated,  then 
the  increase  next  following  will  be  lengthened  or 
contracted  to  a  corresponding  extent.  In  conse- 
quence of  this  we  are  sometimes  able  to  predict 
with  fair  accuracy  when  the  next  ensuing  maxi- 
mum or  minimum  will  take  place. 

The  most  striking  feature  of  such  a  curve  as 
that  just  alluded  to  is  the  very  sensible  secondary 
augmentation  which  happens  very  soon  after  the 
principal  maximum. 

A  very  curious  circumstance  has  come  to  light 
in  connection  with  the  epochs  of  maxima  and 
minima.  In  arranging  the  spots  according  to 
their  latitude  and  longitude  on  a  diagram  suffi- 
ciently contracted,  Carrington  found  that  their 
latitude  decreases  gradually  as  the  period  of 
minimum  draws  near  ;  then  when  their  number 


50        THE  STORY  OF  THE  SOLAR  SYSTEM. 

begins  to  increase  they  begin  to  appear  again  at 
a  higher  latitude.  This  seems  to  be  a  definite 
law.  At  any  rate  Carrington's  conclusion  has 
been  found  to  hold  good  by  the  observations  of 
Sporer  and  Secchi. 

The  variations  of  the  spots  which  we  now  rec- 
ognise naturally  recall  those  obscurations  of  the 
Sun  which  are  recorded  in  history  ;  but  it  is  ne- 
cessary to  accept  many  of  these  with  caution.  A 
great  number  of  these  phenomena  which  attracted 
the  attention  of  people  in  early  times  are  only 
eclipses  badly  observed  and  still  more  badly  de- 
scribed. In  other  instances  the  obscuration  has 
been  produced  by  very  protracted  dry  fogs.  It  is 
probably  to  this  last-named  cause  that  we  must 
ascribe  the  obscuration  which,  according  to  Kep- 
ler and  Gemma  Frisius,  took  place  in  1547. 

It  was  in  some  such  way  as  this  that,  according 
to  Virgil  (Georg.  i,  630),  who  has  echoed  a  tradition 
which  he  found  in  history,  the  Sun  was  obscured 
at  the  death  of  Caesar  :— 

Hie  etiam  extincto  miseratus  Csesare  Romam 
Quum  caput  obscura  nitidum  ferrugine  texit, 
Impiaque  seternam  timuerunt  saecula  noctem. 

In  the  year  553  A.  D.,  and  again  in  the  year 
626  A.  D.  the  Sun  remained  obscured  for  several 
months;  but  these  facts  (if  facts  they  are)  besides 
being  ill-observed,  and  clothed,  no  doubt,  in  ex- 
tremely exaggerated  language,  are  brought  to  our 
notice  as  having  occurred  at  epoch?  which  are 
quite  independent  of  one  another,  whilst  the 
variations  in  the  markings  on  the  Sun,  which  we 
have  just  been  talking  about,  present  an  almost 
mathematical  regularity  of  sequence. 

We  must  now  institute  some  inquiries  as  to 


THE  SUN.  51 

the  causes  of  the  periodicity  of  the  spots.  A 
periodicity  so  well  established  would  naturally 
invite  astronomers  to  seek  the  causes  which  pro- 
duced it.  The  presence  of  spots  only  in  the  Zo- 
diacal regions  led  Galileo  to  suspect  the  existence 
of  some  relation  between  the  spots  and  the  posi- 
tion of  the  planets;  but  there  is  in  this  a  mere 
surmise,  which,  when  it  was  made,  had  nothing  to 
justify  it,  and  it  is  still  impossible  for  us  to  say 
anything  for  certain  on  the  point.  The  deter- 
mining cause  of  the  periodicity  may  exist  in  the 
interior  of  the  Sun,  and  may  depend  on  circum- 
stances which  will  for  ever  remain  unknown  to 
us.  Or  it  may  be  something  external  :  it  may  be 
due  after  all  to  the  influence  of  the  planets.  It 
remains  for  us,  therefore,  to  search  and  see  if  any 
such  influence  can  be  traced. 

According  to  Wolf,  the  attraction  of  the  planets, 
or  of  some  of  them,  is  the  real  cause  of  the  pe- 
riodicity which  we  are  dealing  with ;  that  attrac- 
tion producing  on  the  surface  of  the  solar  globe 
true  tides,  which  give  birth  to  the  spots,  these 
tides  themselves  experiencing  periodic  variations 
owing  to  the  periodic  changes  of  position  of  the 
celestial  bodies  which  cause  them.  It  has  even 
been  thought  safe  to  assert  that  the  fact  of  the 
principal  period  coinciding  with  the  revolution  of 
Jupiter  is  of  momentous  significance;  but  this 
coincidence  seems  purely  accidental,  and  no  cer- 
tain conclusion  can  be  drawn  as  to  this  matter. 
The  influence  of  Mercury  and  Venus  would  per- 
haps be  much  more  potent,  for  their  distance  from 
the  Sun  is  not  very  great,  and  this  should  render 
their  influence  more  sensible.  On  the  other  hand, 
their  masses  appear  to  be  too  small  to  be  capable 
of  producing  any  sufficient  effect, 


52  THE  STORY  OF   THE  SOLAR  SYSTEM. 

De  La  Rue,  Balfour  Stewart,  and  Lowy  most 
perseveringly  studied  this  point  of  solar  physics. 
They  seem  to  have  arrived  at  the  conclusion 
that  the  conjunctions  of  Venus  and  Jupiter  do 
exercise  a  certain  amount  of  influence  on  the 
number  of  the  spots  and  on  their  latitude ;  and 
that  this  influence  is  less  considerable  when  Venus 
is  situated  in  the  plane  of  the  solar  equator. 
At  any  rate  it  is  a  fact,  that  a  great  number  of  the 
visible  inequalities  in  a  duly  plotted  curve  of  the 
spots  do  really  correspond  to  special  positions  of 
these  two  planets. 

In  order  to  determine  with  more  precision 
these  coincidences  and  the  importance  which  at- 
taches to  them,  De  La  Rue  extended  his  inquiries. 
He  separately  analysed  many  different  groups  of 
spots,  selecting  for  his  purpose  more  particularly 
those  of  which  the  observations  happened  to 
have  been  specially  continuous  and  complete, 
giving  a  preference  moreover  to  those  which  had 
been  observed  in  the  central  portions  of  the  Sun's 
disc.  From  an  investigation  of  794  groups  De 
La  Rue  arrived  at  the  following  conclusions : — 
(i)  If  we  take  a  meridian  passing  through  the 
middle  of  the  disc  and  represented  by  a  diameter 
perpendicular  to  the  equator,  we  find  that  the 
mean  size  of  the  spots  is  not  the  same  with  re- 
gard *to  that  meridian.  It  appears  certain  that 
the  correction  required  for  perspective  does  not 
suffice  to  explain  this  difference  ;  and  that  another 
element  must  be  introduced  in  order  to  secure 
that  the  apparent  dimensions  of  the  spots  may 
be  the  same  on  both  sides.  We  do  not  yet  pos- 
sess a  very  clear  explanation  of  this  fact;  but 
the  most  probable  is  this : — the  spots  are  sur- 
rounded by  a  projecting  bank,  which  seems  to 


THE  SUN.  53 

disappear  in  part  during  their  transit  across  the 
Sun.  This  bank  is  more  elevated  on  the  pre- 
ceding than  on  the  following  side ;  accordingly, 
the  spots  ought  to  seem  smaller  when  they  are 
in  the  eastern  half  of  the  disc;  larger  when  they 
are  in  the  western  half ;  for  in  the  first  position 
the  observer's  eye  meets  an  elevated  obstacle, 
which  hides  a  portion  of  the  spot  itself.  (2)  De 
La  Rue  specially  studied  the  spots  observed  at 
the  times  when  the  planets  Venus  and  Mars  were 
at  a  heliocentric  distance  from  the  Earth  equal'to 
o,  90,  180,  and  270  degrees,  and  arrived  at  this 
result ;  the  spots  are  larger  in  the  part  of  the  Sun 
which  is  away  from  Venus  and  Mars,  and  they 
are  smaller  on  the  side  on  which  these  planets 
happen  to  be.  The  same  result  was  obtained, 
whether  Carrington's  figures  or  the  Kew  photo* 
graphs  were  employed.  (3)  Meanwhile  it  does 
not  appear  that  Jupiter  emits  any  similar  influ- 
ence. This  influence  should  be  easily  perceived, 
for  if  we  calculate  the  action  of  the  planets  in  the 
way  that  we  calculate  the  tides,  treating  it  as  di- 
rectly proportional  to  the  masses  and  inversely 
proportional  to  the  cubes  of  the  distances,  the 
influence  of  Jupiter  should  greatly  outweigh  that 
of  Venus. 

Wolf  thought  that  he  had  noticed  traces  of 
some  influence  being  exerted  by  Saturn;  but  this 
remains- altogether  without  confirmation. 

De  La  Rue  noticed  that  large  spots  are  gener- 
ally situated  at  extremities  of  the  same  diameter. 
This  law  also  often  applies  to  the  development 
of  large  prominences.  The  coincidence  agrees 
well  with  the  theory  that  there  exists  on  the  Sun 
some  action  resembling  that  of  our  tides. 

may  be  the  amount  of  probability 


54  THE  STORY  OF  THE  SOLAR  SYSTEM. 

which  attaches  to  these  explanations  we  ought 
not  to  forget  that  we  are  still  far  off  from  pos- 
sessing the  power  of  giving  a  vigorous  demon- 
stration of  them.  If  we  consider  with  attention 
the  periodical  variations  of  the  spots  we  shall  not 
be  long  in  coming  to  the  conclusion  that  it  is  im- 
possible to  connect  them  directly  with  any  one 
astronomical  function  in  particular,  for  the  spots 
appear  in  a  sudden  and  irregular  manner  which 
contrasts  in  a  striking  degree  with  the  continuous 
and.  progressive  action  of  the  ordinary  perturba- 
tions which  we  meet  with  in  the  study  of  Celestial 
Mechanics.  There  is  but  one  reply  possible  to 
this  objection.  The  spots  and  their  changes  must 
be  visible  manifestations  of  the  periodical  activity 
of  the  Sun — an  activity  which  itself  depends  (as 
assumed)  on  the  action  of  the  planets  and  on 
their  relative  positions.  The  cause,  thus  defined, 
of  the  Sun's  activity  may  be  very  regular  ;  the 
activity  itself  may  vary  in  a  continuous  manner 
without  the  resulting  phenomena  possessing  the 
same  continuity  and  the  same  regularity.  We  see 
this  in  the  periodical  succession  of  the  Seasons  on 
the  Earth.  The  position  of  the  Sun,  and  conse- 
quently its  manner  of  acting  upon  our  globe, 
varies  with  a  remarkable  uniformity,  but  never- 
theless the  meteorological  phenomena  which  re- 
sult are  irregular  and  capricious.  Thus  it  comes 
about  that  physicists  are  more  and  more  inclined 
to  believe  that  the  spots  are  only  secondary 
effects  produced  by  causes  more  important  and 
more  fundamental. 

Whatever  may  be  our  ignorance  as  to  the 
causes  which  produce  variations  in  the  Sun's  activ- 
ity we  may  at  least  draw  one  conclusion  from 
the  preceding  remarks :  it  is,  that  the  Sun  is  a 


THE  SUN.  55 

very  long  way  from  having  arrived  at  a  state  of 
tranquillity  and  freedom  from  internal  commotion. 
On  the  contrary,  it  is  the  seat  of  great  movements. 
Its  activity  is  subject  to  numberless  periodical 
changes  which  ought  in  their  turn  to  influence  the 
intensity  of  the  heat  and  light  given  out  by  the 
Sun ;  and  so  re-act  on  the  planets  which  receive 
their  heat,  light,  and  life  from  the  Sun. 

No  account  of  the  periodicity  of  the  spots  on 
the  Sun  can  be  deemed  complete  which  does  not 
include  information  respecting  certain  other  peri- 
odical phenomena  which  have  been  found  to  ex- 
hibit features  of  alternation  closely  resembling 
in  their  sequence  and  character  the  periodical 
changes  which  take  place  in  regard  to  the  spots 
on  the  Sun.  There  is  evidently  a  deep  mystery 
lying  hid  under  the  curious  fact  (which  is  clearly 
established)  that  the  n-year  period  of  the  spots 
coincides  in  a  manner  as  unexpected  as  it  is  cer- 
tain with  the  period  of  the  variation  of  terrestrial 
magnetism.  The  magnetic  needle  is  subject  to 
a  diurnal  variation  which  reaches  its  extreme 
amount  every  n  years,  and  not  only  so,  but  the 
epoch  of  maximum  variation  corresponds  with  the 
epoch  of  the  maximum  prevalence  of  Sun  spots. 
And  similarly  years  in  which  the  needle  is  least 
disturbed  are  also  years  in  which  the  Sun  spots 
are  fewest.  Two  other  very  curious  discoveries 
have  also  been  made  which  are  in  evident  close 
connection  with  the  foregoing.  The  manifesta- 
tion of  the  Aurora  Borealis  and  of  those  strange 
currents  of  electricity  known  as  magnetic  earth 
currents  (which  travel  below  the  Earth's  surface 
and  frequently  interfere  with  telegraphic  opera- 
tions), likewise  exhibit  periodical  changes  which 
take  1 1  years  to  go  through  all  their  stages.  This 


56  THE  STORY  OF  THE  SOLAR  SYSTEM. 

fact  alone  would  be  sufficiently  curious,  but  when 
we  come  to  find  that  the  curve  which  exhibits  the 
changes  these  two  manifestations  of  force  go 
through,  also  shows  that  their  maxima  and  min- 
ima are  contemporaneous  with  the  maxima  and 


FIG.  8.— The  Sun  totally  eclipsed,  July  18,  1860  (Feilitzsch). 

minima  of  the  Sun  spots  and  magnetic  needle 
variations,  we  cannot  doubt  that  (to  use  Balfour 
Stewart's  words)  "  a  bond  of  union  exists  between 
these  four  phenomena.  The  question  next  arises, 
what  is  the  nature  of  this  bond  ?  Now,  with  re- 
spect to  that  which  connects  Sun  spots  with  mag- 
netic disturbances  we  can  as  yet  form  no  conjec- 
ture." To  cut  a  long  story  short,  it  may  be  said 
generally  that  whilst  without  doubt  electricity  is 
the  common  basis  of  the  three  last-named  of  the 
four  phenomena  just  mentioned,  it  seems  scarcely 


MERCURY.  57 

too  great  a  stretch  of  the  imagination  to  go  one 
step  further  and  suggest  that  electricity  has  in 
some  or  other  occult  manner  something  to  do 
with  all  these  things  and  therefore  with  the  spots 
on  the  Sun. 

The  reader  who  has  followed  me  thus  far  will 
by  this  time  be  in  a  position  to  appreciate  a  re- 
mark made  in  an  earlier  part  of  this  chapter,  that 
the  multitude  of  facts  known  to  us  in  connection 
with  the  Sun  and  its  spots  is  so  great,  as  to  render 
it  impossible  to  exhibit  in  a  single  chapter  any- 
thing more  than  the  barest  outline  of  them.  The 
numerous  observations  of  recent  eclipses  of  the 
Sun,  especially  since  that  of  1860,  and  the  exten- 
sive application  of  the  spectroscope  to  the  Sun 
both  in  connection  with  these  eclipses,  and  gen- 
erally, may  be  said  to  have  completely  revolu- 
tionised our  knowledge  of  solar  phenomena  dur- 
ing the  present  generation  ;  or  perhaps  it  might 
be  more  correct  to  say  have  enormously  increased 
our  knowledge  of  the  facts  of  the  case  and  have 
revolutionised  in  no  small  degree  the  conclusions 
deduced  from  the  facts. 


CHAPTER   III. 

MERCURY. 

So  far  as  we  know  at  present,  Mercury  is  the 
nearest  planet  to  the  Sun.  The  circumstances 
under  Vhich  it  presents  itself  to  us  and  a  brief 
general  account  of  its  movements  have  already 
been  stated.  In  the  present  chapter,  therefore 
(and  this  remark  applies  in  substance  to  each  of 


58  THE  STORY  OF  THE  SOLAR  SYSTEM. 

the  succeeding  chapters  appropriated  to  particular 
planets),  I  shall  limit  myself  to  such  topics  as 
seem  to  be  of  interest  to  an  observer  armed  with 
a  telescope.  Mercury,  as  already  mentioned,  ex- 
hibits from  time  to  time  phases  which  may  be 
said  to  be  the  same  as  those  of  the  moon;  but  as 
the  only  chance  of  seeing  it  is  when  it  is  at  its 
greatest  distance  east  or  west  of  the  Sun,  practi- 
cally it  can  only  be  studied  when  in,  or  rather 
near  to,  what  may  be  called  the  half-moon  phase ; 
and  even  then  observations  on  its  physical  ap- 
pearance can  only  be  obtained  with  difficulty. 
Perhaps  its  most  definite  feature  is  its  colour. 
This,  undoubtedly,  is  more  or  less  pink.  Strange 
to  say,  in  spite  of  the  multiplication  of  telescopes 
and  observers,  comparatively  little  attention  has 
been  paid  to  this  planet,  and  we  really  know  very 
little  more  about  it  than  Schroter  told  us  nearly 
a  hundred  years  ago.  He  obtained  what  he  con- 
ceived to  be  satisfactory  evidence  of  the  existence 
of  at  any  rate  one  mountain,  having  a  height  of 
about  ii  English  miles — a  height  which  it  will  be 
noted,  far  exceeds,  not  only  relatively  but  abso- 
lutely, any  mountain  on  the  earth.  What  Schro- 
ter based  this  conclusion  upon  was  the  fact  that 
when  the  planet  was  near  inferior  conjunction, 
the  southern  horn  presented  a  truncated  appear- 
ance, which  might  be  the  result  of  a  lofty  projec- 
tion arresting  the  Sun's  light.  Schroter  also  an- 
nounced that  Mercury  rotated  on  its  axis  in  24 
hours  5  minutes.  Sir  W.  Herschel  failed  to  satisfy 
himself  that  Schroter's  conclusions  were  well- 
founded,  but  it  must  certainly  be  admitted  that 
some  support  for  them  is  furnished  by  certain  ob- 
servations made  within  the  last  few  years.  It  is 
matter  for  regret,  however,  that  most  of  these 


MERCURY.  59 

were  made  with  instruments  of  sizes  which,  for 
the  most  part,  cannot  be  said  to  have  been  equal 
to  the  task  to  which  they  were  applied.  The 
truncature  of  the  southern  horn  first  spoken  of  by 
Schroter,  was  thought  by  Denning,  in  1882,  to  be 
obvious ;  and  in  the  same  year,  by  watching  the 
displacement  of  certain  bright  and  dusky  spaces 
on  the  disc,  the  same  observer  concluded  that  a 
rotation  period  of  about  25  hours  was  indicated. 

In  1882  Schiaparelli  at  Milan  commenced  a 
prolonged  study  of  Mercury.  Believing  that  it 
was  essential  to  observe  through  a  good  condition 
of  atmosphere,  and  that  this  was  impossible  if  the 
planet  were  only  looked  at  in  twilight,  when  it 
was  necessarily  at  a  low  altitude,  Schiaparelli 
made  all  his  observations  with  the  Sun  and  planet 
high  up  in  the  heavens.  He  considered,  in  effect, 
that  the  blaze  of  the  Sun's  light  was  a  lesser  evil 
than  the  tremors  inseparable  from  observations  of 
the  planet,  clear  it  might  be  in  some  degree  of 
•inconvenient  Sun-light,  but  viewed  through  the 
vapours  and  atmospheric  disturbances,  which  al- 
ways spoil  all  observations  near  the  horizon. 
Schiaparelli's  observations  yielded  various  results, 
most  of  them  novel,  and  one  of  them  very  star- 
tling. He  considers  Mercury  to  be  a  much  spotted 
globe  and  to  be  enveloped  in  a  tolerably  dense 
atmosphere.  He  thought  he  noticed  brownish 
stripes  and  streaks  (which  might  be  regarded  as 
permanent  markings),  more  clearly  visible  on 
some  occasions  than  on  others;  and  that  these 
systematically  disappeared  near  the  limb,  owing 
to  the  increased  depth  there  of  the  atmosphere 
through  which  they  had  to  be  looked  at. 

The  foregoing  observations  may  be  regarded 
as  not  unreasonable ;  they  may  even  be  accepted 


60  THE  STORY  OF   THE  SOLAR  SYSTEM. 

without  further  question.  But  what  are  we  to  say 
to  Schiaparelli's  conclusions  that  these  markings 
are  so  nearly  permanent,  taking  one  day  with  an- 
other, that  Mercury's  rotation  cannot  be  meas- 
ured in  hours  at  all,  but  is  a  matter  of  days, — in 
point  of  fact,  of  88  days;  and  that  in  reality  Mer- 
cury occupies  in  its  rotation  on  its  axis  the  whole 
of  the  88  days  which  constitute  its  sidereal  year, 
or  period  of  revolution  round  the  Sun.  The  coun- 
terpart of  this  for  us  would  be  that,  instead  of  the 
inhabitants  of  the  earth  having  a  day  of  24  hours, 
they  would  have  only  one  day  and  night  every 
365  days.  Astronomers  are  not  at  present  satis- 
fied to  accept  this  conclusion  in  regard  to  Mercury. 
Some  observers  have  thought  that  Mercury  is 
more  easy  to  observe  than  Venus,  and  that,  speak- 
ing generally,  its  surface,  if  we  could  only  get  to 
see  it  constantly  under  favourable  circumstances, 
might  be  considered  to  resemble  in  most  respects 
that  of  Mars.  Mercury  revolves  round  the  Sun  at 
a  mean  distance  of  36  millions  of  miles.  Owing, 
however,  to  the  fact  that  the  eccentricity  of  its 
orbit  (or  its  departure  from  the  circular  form) 
is  greater  than  that  of  any  of  the  other  major 
planets,  it  may  approach  to  within  28-^  millions  of 
miles  or  recede  to  more  than  43  millions  of  miles. 
Its  apparent  diameter  varies  between  4^"  in  su- 
perior conjunction  to  13"  in  inferior  conjunction. 
The  real  diameter  may  be  taken  at  about  3000 
miles. 


VENUS.  6 1 

CHAPTER   IV. 

VENUS. 

THE  planet  Venus  has  two  things  in  common 
with  Mercury.  One  is,  that  being  an  inferior 
planet,  that  is  to  say,  a  planet  revolving  round 
the  Sun  in  an  orbit  within  that  of  the  Earth,  it  is 
never  very  far  distant  from  the  Sun,  and  there- 
fore can  never  be  seen  on  a  distinctly  dark  sky. 
The  second  point  alluded  to  arises  out  of  the 
first;  Venus  exhibits  from  time  to  time  a  series 
of  phases  which  are  identical  in  character  with 
those  of  Mercury,  and  therefore  with  those  of  the 
Moon.  Venus  differs,  however,  from  Mercury  in 
the  very  important  point  of  size.  Inasmuch  as 
its  diameter  is  considerably  more  than  double  the 
diameter  of  Mercury  it  has  a  surface  more  than 
six  times  as  great,  and  therefore  exhibits  a  far 
larger  area  of  illumination  than  Mercury  does. 
The  result  of  this  (coupled  with  another  fact 
which  will  be  stated  presently)  is  that  the  planet 
may  often  be  easily  seen  in  broad  daylight,  and 
sometimes  casts  a  sensible  shadow  at  night.  Un- 
der special  circumstances,  which  recur  every  8 
years,  this  planet  shines  with  very  peculiar  bril- 
liancy. True,  that  only  about  Jth  of  the  whole 
disc  is  then  illuminated,  but  that  fraction  trans- 
mits to  us  more  light  than  phases  of  greater  ex- 
tent do,  because  these  latter  coincide  with  epochs 
when  the  planet  is  more  remote  from  the  Earth. 

Spots  and  shadings  have  on  various  occasions 
been  noticed  on  Venus,  and  though  it  is  not  easy 
to  harmonise  the  various  accounts,  there  seems 
no  doubt  of  the  reality  of  the  facts,  or  that  they 


62  THE  STORY  OF  THE  SOLAR  SYSTEM. 

must  be  ascribed  to  the  existence  of  mountains. 
Schroter  found  very  much  the  same  state  of 
things  to  exist  on  Venus  that  he  found  on  Mer- 
cury, and  putting  together  what  he  saw  he  ar- 
rived at  the  conclusion  that  Venus  possesses 
mountains  of  considerable  height,  and  that  his 
observations  must  be  taken  to  imply  that  the 
planet  revolved  on  its  axis  in  rather  more  than 
23  hours.  This  conclusion  as  regards  the  planet's 
axial  rotation  was  not  first  arrived  at  by  Schroter, 
for  the  two  Cassinis,  one  about  1666,  and  the 
other  about  1740,  both  ascribed  to  Venus  a  rota- 
tion period  of  about  23  hours,  an  evaluation  which 
was  fully  confirmed  by  Di  Vico  at  Rome  between 
1839  and  1841,  and  by  Flammarion  in  1894. 

What  has  been  already  said  with  respect  to 
Mercury  is  true  also  of  Venus,  namely  that  it  has 
been  much  neglected  by  modern  observers;  and 
accordingly  an  announcement  made  by  Schia- 
parelli  in  1890,  that  the  rotation  period  of  Venus 
is  to  be  measured  not  by  hours  but  by  months, 
came  upon  the  astronomical  world  as  a  startling 
revelation  ;  but  it  is  a  revelation  which  has  been 
keenly  contested,  and  certainly  awaits  legal  proof. 
Schiaparelli  has  not  ventured  to  assert  as  he  has 
done  in  the  case  of  Mercury,  that  Venus's  rotation 
period  is  identical  with  the  period  of  7^  months 
in  which  it  revolves  round  the  Sun ;  he  only 
claims  this  as  a  strong  probability  arising  out  of 
what  he  says  he  is  certain  of,  namely  that  its 
period  of  rotation  cannot  be  less  than  six  months 
and  may  be  as  much  as  nine  months.  His  as- 
sumption is  that  previous  observers  in  endeav- 
ouring to  ascertain  Venus's  rotation  period  have 
used  and  relied  upon  evanescent  shadings  which 
probably  were  of  atmospheric  origin  and  scarcely 


VENUS.  63 

recognisable  from  day  to  day,  whereas  he  fixed 
his  attention  upon  round  defined  white  spots, 
which,  whatever  their  origin,  are  so  far  permanent 
that  their  existence  has  been  spoken  of  for  two 
centuries.  Miss  Clarke  thus  puts  the  matter: — 
"  His  steady  watch  over  them  showed  the  in- 
variability of  their  position  with  regard  to  the 
terminator;  and  this  is  as  much  as  to  say  that  the 
regions  of  day  and  night  do  not  shift  on  the  sur- 
face of  the  planet.  In  other  words  she  keeps  the 
same  face  always  turned  towards  the  Sun." 

Various  recent  observations,  some  of  them 
made  with  the  express  object  of  throwing  light 
upon  Schiaparelli's  conclusions,  are  strangely  con- 
tradictory. Perrotin  at  Nice  in  1890  thought  his 
observations  confirmed  Schiaparelli's  ;  on  the  other 
hand  Niesten  at  Brussels  considered  that  numer- 
ous drawings  of  Venus  made  by  himself  and 
Stuyvaert  between  1881  and  1890  harmonised 
well  with  DiVico's  rotation  period  of  23?!.  2im. 
22S. ;  which  Trouvelot  in  1892  only  wished  to  in- 
crease to  about  24  hours. 

There  is  a  general  consensus  of  opinion  that 
great  irregularities  exist  on  the  surface  of  Venus. 
These  are  made  specially  manifest  to  us  in  con- 
nection with  the  terminator  or  visible  edge  of  the 
planet  seen  as  an  illuminated  crescent.  If  the 
planet  had  a  smooth  surface  this  line  would  at  all 
times  be  a  perfect  and  continuous  curve,  instead 
of  which  it  is  frequently  to  be  noticed  as  a  jagged 
or  broken  line.  Observations  to  this  effect  go 
back  as  far  as  1643,  when  Fontana  at  Naples  ob- 
served this  to  be  the  condition  of  the  terminator. 
La  Hire,  Schroter,  Madler,  Di  Vico  and  many 
others  down  to  the  present  epoch  have  noted  the 
same  thing.  The  fact  that  the  southern  horn  of 
5 


64 


THE   STORY  OF   THE   SOLAR  SYSTEM. 


Venus  is^constantly  to  be  seen  blunted  is  so  well 
established    as  to  admit  of   no   doubt,   and  this 

blunting  is 
commonly  as- 
cribed to  the 
existence  of  a 
lofty  moun- 
tain, to  which 
Schroter  as- 
cribed a  height 
of  27  miles. 
Whatever  we 
may  think  as 
to  the  precise 
accuracy  of 
this  figure,  it 
seems  impos- 
sible to  doubt 
the  main  fact 
on  which  it  de- 
pends ;  whilst 
a  Belgian  observer,  Van  Ertborn,  in  1876  repeat- 
edly saw  a  point  of  light  in  this  locality  which  he 
regarded  as  due  to  Sun-light  impinging  on  a  de- 
tached peak,  adjacent  valleys  remaining  in  shadow. 
This  effect  is  common  enough  in  the  case  of  the 
Moon,  and  is  familiar  to  all  who  are  in  the  habit 
of  studying  the  Moon. 

The  existence  on  Venus  of  an  atmosphere  of 
considerable  density  and  extent  is  well  estab- 
lished. Proof  of  this  is  to  be  found  in  the 
marked  diminution  of  the  planet's  brilliancy 
towards  the  terminator;  and  in  the  faint  curved 
line  of  light  which  occasionally  may  be  seen  when 
the  planet  is  near  inferior  conjunction.  When  so 
situated,  so  much  of  the  planet  itself  as  can  be 


FIG.  9. — Venus,  Dec.  23,  1885. 


VENUS. 


seen  illuminated  shows  as  a  narrow  radiant  cres- 
cent of  light,  ending  off  in  two  points  called  in- 
differently cusps  or  horns.  It  sometimes  happens, 
however,  that  from  the  point  of  each  cusp  there 
runs  round  to  the  other  cusp  a  faint  continuation 
of  the  crescent,  resulting  in  the  general  appear- 
ance of  the  planet  being  that  of  a  nearly  uniform 
ring  of  light. 
There  is  no 
known  way  in 
which  the  Sun 
can  illuminate 
so  much  more 
than  the  half 
of  Venus  so  as 
to  permit  of  a 
perfect  circle 
being  visible 
except  by  sup- 
posing that 
an  atmosphere 
exists  on  the 
planet  and 
refracts  (or 
transmits  by  bending,  as  it  were,  round  the  cor- 
ner) a  sufficient  amount  of  Sun-light  to  give  rise 
to  the  appearance  in  question.  Further  proof  of 
the  existence  of  an  atmosphere  on  Venus  is  ob- 
tainable on  those  very  rare  occasions  when  the 
planet  is  seen  passing  across  the  disc  of  the  Sun 
— a  phenomenon  known  as  a  "  Transit  of  Venus." 
It  then  nearly  always  happens  that  a  hazy  nebu- 
lous ring  of  feeble  light  may  be  detected  encom- 
passing the  planet's  disc  indicative  of  course  of 
the  fact  that  the  Sun's  rays  are  there  slightly 
obstructed  in  reaching  the  eye  of  an  observer  on 


FIG.  10. — Venus  near  conjunction  as  a  thin 
crescent,  Sept.  21,  1887  (Flammarion). 


66  THE  STORY  OF   THE  SOLAR  SYSTEM. 

the  Earth.  Some  observers  scrutinising  Venus 
when  in  transit  have  thought  that  they  were  able 
to  obtain,  by  means  of  the  spectroscope,  traces  of 
aqueous  vapour  on  the  planet,  but  the  evidence 
of  this  does  not  appear  to  be  altogether  clear  or 
conclusive. 

Everybody  may  be  presumed  to  be  acquainted 
with  the  spectacle  popularly  known  as  "  The  Old 
Moon  in  the  New  Moon's  Arms  "  whereby  when 
the  Moon  is  only  about  two  or  three  days  old  and 
exhibits  but  a  narrow  crescent  of  bright  light,,  yet 
the  whole  outline  of  the  disc  is  traceable  on  the 
•sky.  A  phenomenon  analogous  to  this  may  often 
be  seen  in  the  case  of  Venus  when  near  its  infe- 
rior conjunction.  With  the  Moon  the  cause  is 
due  to  the  reflection  of  Earth-light  (so  to  speak) 
to  the  Moon,  but  that  explanation  seems  inade- 
quate in  respect  of  Venus,  because  it  is  conceived 
that  the  amount  of  Earth-light  available  is  alto- 
gether insufficient  for  the  purpose.  Many  other 
explanations  have  been  put  forward  including 
phosphorescence  on  the  surface  of  Venus,  elec- 
trical displays  in  the  nature  of  terrestrial  aurorae, 
and  what  not,  but  it  must  be  frankly  confessed 
that  astronomers  are  all  at  sea  on  the  subject. 

The  existence  of  snow  at  the  poles  of  Venus 
has  been  suspected  by  observers  of  tried  skill 
and  experience  such  as  Phillips  and  Webb,  though 
the  idea  was  first  broached  by  Gruithuisen  in 
1813.  Flammarion's  observations  during  1892 
and  the  two  following  years  are  distinctly  con- 
firmatory of  this  idea.  He  adds  that  as  both 
polar  caps  are  visible  at  the  same  time  the  plan- 
et's axis  cannot  be  much  inclined  to  the  plane  of 
its  orbit. 

Compared  with  all  the  other  planets  the  ab- 


VENUS.  67 

solute  brightness  of  Venus  stands  very  high.  Of 
course  it  must  be  understood  that  by  this  phrase 
"absolute  brightness"  no  more  is  meant  than  its 
reflective  power.  Venus  is  what  it  is  by  virtue  of 
its  power  of  reflecting  Sun-light;  presumably  it 
has  no  inherent  brightness  of  its  own.  What  iU 
reflective  power  is  was  probably  never  more 
effectively  brought  under  the  notice  of  a  human 
eye  than  on  September  26,  1878,  when  Nasmyth 
enjoyed  an  opportunity  of  seeing  Venus  ana 
Mercury  side  by  side  for  several  hours  in  the 
same  field  of  view.  He  speaks  of  Venus  as  re- 
sembling clean  silver  and  Mercury  as  nothing 
better  than  lead  or  zinc.  Seeing  that  owing  to 
its  greater  proximity  to  the  Sun  the  light  incident 
on  Mercury  must  be  some  3^  times  as  strong  as 
the  light  incident  on  Venus,  it  follows  that  the 
reflective  power  of  Venus  must  be  very  great. 
As  a  matter  of  fact  it  has  been  calculated  to  be 
nearly  equal  to  newly  fallen  snow;  in  other  words 
to  reflect  fully  70  per  cent,  of  the  light  which 
impinges  on  it. 

Venus  has  no  satellite;  this  fact  seems  certain. 
Yet  half  a  dozen  or  more  observers  between  1645 
and  1768  discovered  such  a  satellite;  observed  a; 
followed  it!  This  startling  mystery,  as  it  realiy 
was,  attracted  some  years  ago  the  attention  of  a 
very  careful  Belgian  observer,  Stroobant,  wno 
examined  in  a  most  painstaking  manner  all  me 
recorded  observations.  His  conclusions  were 
that  in  almost  all  cases  particular  stars  (which  he 
identified)  were  mistaken  for  a  satellite.  Where 
the  object  seen  was  not  capable  of  identification, 
possibly  it  was  a  minor  planet ;  whilst  in  one  in- 
stance it  was  probable  that  it  was  Uranus  which 
had  been  seen  and  regarded  as  a  satellite  of  Venus. 


68  THE   STORY  OF  THE  SOLAR  SYSTEM. 

Venus  is  perhaps  the  planet  which  has  most 
impressed  the  popular  mipd.  For  the  earliest 
illustration  of  this  statement  we  must  go  as  far 
back  as  Homer  who  makes  two  references  to  it 
in  the  Iliad.  These,  in  Pope's  version,  run  as 
follows : — 

"  As  radiant  Hesper  shines  with  keener  light, 
Far  beaming  o'er  the  silver  host  of  night." 

— xxii.  399  [318]. 

"  The  morning  planet  told  th'  approach  of  light  ; 
And  fast  behind,  Aurora's  warmer  ray 
O'er  the  broad  ocean  pour'd  the  golden  day." 

— xxiii.  281  [226]. 

The  phases  of  Venus  were  first  discovered  by 
Galileo  and  were  made  known  to  the  world,  or 
rather  to  Kepler,  in  a  mystic  sentence  which  has 
often  been  quoted  : — 

"  Hac  immatura,  a  me  jam  frustra  leguntur — oy" 

"  These  things  not  ripe  ;  at  present  [read]  in  vain   [by  others] 
are  read  by  me." 

The  former  sentence  transposed  becomes — 

Cynthia  figuras  cemulatur  mater  amorum. 

The  mother  of  loves  [Venus]  imitates  the  phases  of  Cynthia 
[the  Moon]. 

Venus  revolves  round  the  Sun  in  224^  days  at 
a  mean  distance  of  about  67  millions  of  miles. 
Its  apparent  diameter  varies  between  p-J-"  in  su- 
perior conjunction,  and  62"  in  inferior  conjunc- 
tion. The  real  diameter  is  about  7500  miles;  ib 
other  words  Venus  is  nearly  as  large  as  the 
Earth. 


THE  EARTH.  69 


CHAPTER  V. 

THE   EARTH. 

To  us,  as  its  inhabitants,  the  Earth  appeals  in 
two  characters,  and  in  writing  a  book  on  astron- 
omy it  is  necessary,  yet  difficult,  to  keep  these 
two  characters  separate.  The  Earth  is  an  ordi- 
nary planet  member  of  the  solar  system,  amenable 
to  the  same  laws,  impelled  by  the  same  forces, 
and  going  through  the  same  movements  as  the 
other  members  of  the  Sun's  entourage.  Yet,  by 
reason  of  the  fact  that  we  are  ourselves  on  the 
Earth  and  are  not  spectators  of  it  looking  at  it 
from  at  a  distance,  there  are  many  phenomena 
coming  under  our  notice  which  require  special 
treatment,  and  it  is  often  very  difficult  to  say 
where  the  province  of  the  astronomer  ends  and 
that  of  the  geographer  begins.  This  volume 
being  specially  designed  to  deal  with  astronomical 
matters,  I  shall  pass  over  many  subjects  which 
may  be  said  to  be  on  the  border  line,  and  which 
some  of  my  readers  may  therefore  be  disappointed 
not  to  find  discussed.  Besides  the  geographer, 
the  geologist  and  his  scientific  brother  the  miner- 
alogist are  concerned  with  the  Earth  regarded 
as  a  planet  moving  through  space  as  the  other 
planets  do.  The  geologist  studies  the  actual 
structure  of  the  Earth,  its  circumstances  and  his- 
tory so  far  as  they  have  been  revealed  to  us, 
whilst  the  mineralogist  investigates  and  names 
the  materials  of  which  it  is  composed,  and  classifies 
such  materials  with  the  assistance  of  the  geologist 
on  the  one  hand  and  of  the  chemist  on  the  other. 
All  these  subordinate  sciences — subordinate  I 


70  THE  STORY  OF  THE   SOLAR  SYSTEM. 

mean  from  an  astronomer's  point  of  view — open 
up  very  varied,  instructive,  and  interesting  fields 
of  study,  but  they  are  of  course  foreign  to  the 
purpose  of  the  present  volume. 

Though  the  Earth  is  commonly  regarded  as  a 
sphere  it  is  not  that  in  reality,  because  it  is  not 
of  identical  dimensions  from  east  to  west  and 
from  north  to  south.  It  is  somewhat  flattened 
at  the  poles ;  its  polar  diameter  is  less  than  its 
equatorial  diameter,  in  the  ratio  of  about  298  to 
299,  or,  expressed  in  miles,  its  polar  diameter  is 
about  26  miles  less  than  its  equatorial  diameter. 
If  a  globe  3  feet  in  diameter  be  taken  to  represent 
the  Earth,  then  the  polar  diameter  will,  on  this 
scale,  be  %  inch  too  long.  This  flattening  of  the 
poles  of  the  Earth  finds  its  counterpart,  so  far  as  we 
know,  in  most,  and  probably  in  all  of  the  planets. 
It  is  most  considerable  and  therefore  most  con- 
spicuous in  the  case  of  Jupiter.  It  ought  here  to 
be  added  that  a  suspicion  exists  that  the  equato- 
rial section  of  the  Earth  is  not  a  perfect  circle, 
but  that  the  diameter  of  the  Earth,  taken  through 
the  points  on  the  equator  marked  by  the  merid- 
ians 13°  58'  and  193°  58'  east  of  Greenwich,  is  one 
mile  longer  than  the  diameter  at  right  angles  to 
these  two  points. 

The  science  which  inquires  into  matters  of  this 
kind,  including  besides  the  figure  of  the  Earth, 
the  length  of  the  degree  at  different  latitudes, 
and  the  distances  of  places  from  one  another, 
alike  in  angular  measure  and  in  time,  is  called 
Geodesy ;  it  is,  in  point  of  fact,  land-surveying 
on  a  very  large  scale,  in  which  instruments  and 
processes  of  astronomical  origin  are  brought  into 
operation,  and  in  which  astronomers  are  more  or 
less  required  to  take  the  lead. 


THE   EARTH.  71 

Although  we  all  of  us  now  perfectly  under- 
stand that  the  Earth  is  a  planet  moving  round 
the  Sun  as  a  centre,  it  is,  comparatively  speaking, 
but  recently  that  this  fact  has  become  generally 
recognised  and  understood.  It  is  true  that  we 
can  discover  here  and  there  in  ancient  writings 
some  trace  of  the  idea,  yet  it  is  doubtful  whether 
2000  years  ago  more  than  a  few  "  advanced  " 
thinkers  thoroughly  and  clearly  accepted  it  as 
a  distinct  truth.  It  was  much  more  in  consonance 
with  popular  thought  and  the  actual  appearance 
of  things  that  the  Earth  should  be  the  centre 
round  which  the  Sun  revolved  and  on  which  the 
planets  depended  ;  and  accordingly,  sometimes  in 
one  shape  and  sometimes  in  another,  the  notion 
of  the  Earth  being  the  centre  of  the  universe  was 
generally  accepted.  The  contrary  opinion  had, 
however,  a  few  sympathisers.  For  instance,  Aris- 
tarchus  of  Samos,  who  lived  in  the  third  century 
before  the  Christian  era,  supposed,  if  we  may  trust 
the  testimony  of  Archimedes  and  Plutarch,  that 
the  Earth  revolved  round  the  Sun  ;  this,  however, 
was  regarded  as  a  "  heresy,"  in  respect  of  which 
he  was  accused  of  "impiety."  Some  few  years 
elapsed  and  a  certain  Cleanthes  of  Assos  is  said 
by  Plutarch  to  have  suggested  that  the  great 
phenomena  of  the  universe  might  be  explained 
by  assuming  that  the  Earth  was  endued  with  a 
motion  of  translation  round  the  Sun  together  with 
one  of  rotation  on  its  own  axis.  The  historian 
states  that  this  idea  was  so  contrary  to  the  re- 
ceived opinions  that  it  was  proposed  to  put  Clean- 
thes on  his  trial  for  impiety. 

In  former  times  the  philosophers  who  studied 
the  solar  system  ranged  themselves  in  several 
"  schools  of  thought,"  to  use  a  modern  hackneyed 


72  THE   STORY  OF   THE   SOLAR   SYSTEM. 

phrase.  Some  upheld  the  Ptolemaic  system, 
which  took  its  name  from  a  great  Egyptian  as- 
tronomer, Claudius  Ptolemy,  though  it  does  not 
appear  that  he  was  actually  the  first  to  suggest 
it.  The  Ptolemaic  system  regarded  the  Earth  as 
the  centre,  with  the  following  bodies,  all  called 
planets,  revolving  round  it  in  the  order  stated  : — 
the  Moon,  Mercury,  Venus,  the  Sun,  Mars,  Jupiter, 
and  Saturn.  It  will  be  observed  that  there  are 
seven  bodies  here  named,  and  as  seven  was  re- 
garded as  the  "  number  of  perfection,"  it  was  in 
later  times  considered  that  only  these  seven 
bodies  (neither  more  nor  less)  could  really  be  the 
Earth's  celestial  attendants.  Though  Ptolemy 
was  in  one  sense  an  Egyptian,  there  yet  prevailed 
amongst  the  Egyptians  at  large  another  theory 
slightly  different  from  Ptolemy's.  According  to 
the  "  Egyptian  theory,"  Mercury  and  Venus  were 
regarded  as  satellites  of  the  Sun,  and  not  as  pri- 
mary planets  appurtenant  to  the  Earth. 

After  Ptolemy's  era  many  centuries  elapsed, 
during  which  the  whole  subject  of  the  solar  sys- 
tem lay  practically  dormant,  and  it  continued 
so  until  the  revival  of  learning  brought  new 
theorists  upon  the  scene.  The  most  important 
of  these  was  Copernicus,  who,  in  the  sixteenth 
century,  propounded  a  theory  which  eventually 
superseded  all  others,  and,  with  slight  modifica- 
tions, is  the  one  now  accepted.  Copernicus 
placed  the  Sun  in  the  centre  of  the  system,  and 
treated  it  as  the  point  around  which  all  the  pri- 
mary planets  revolved.  So  far,  so  good ;  but 
Copernicus  went  astray  on  the  question  of  the 
orbits  of  the  planets.  He  failed  to  realise  the 
true  character  of  the  curves  which  they  follow 
and  treated  these  curves  as  "  epicycles,"  which 


THE   EARTH.  73 

word  may  be  described  as  representing  a  compli- 
cated combination  of  little  circles  which  taken 
together  form  a  big  one.  It  was  left  to  Kepler 
and  Newton  to  settle  all  such  details  on  a  true 
and  firm  basis.  But  before  this  stage  was 
reached  a  man  of  the  highest  astronomical  attain- 
ments and  practical  experience,  Tycho  Brahe, 
made  shipwreck  of  his  reputation  as  an  astrono- 
mer by  solemnly  reviving  the  idea  of  the  Earth 
being  the  immovable  centre  of  everything.  He 
treated  the  Moon  as  revolving  round  the  Earth 
at  no  great  distance  and  the  Sun  as  doing  the 
same  thing  a  little  farther  off;  the  five  planets 
revolving  round  the  sun  as  solar  satellites.  The 
"  Tychonic  system,"  as  it  is  called,  has  something 
in  common  with  the  Ptolemaic  system  without 
being  by  any  means  as  logical  as  the  latter. 
That  such  far  fetched  ideas  as  Tycho's  should 
have  been  palmed  off  on  the  world  of  science  so 
recently  as  300  years  ago  is  passing  strange;  but 
the  explanation  appears  to  be  that  his  action 
arose  out  of  a  misconception  of  certain  passages 
of  Holy  Scripture,  which  seemed  irreconcilable 
with  the  Copernican  theory.  It  must  not  be 
forgotten  that  Copernicus's  famous  book,  pub- 
lished in  1543,  in  which  he  had  announced  his 
views,  had  been  condemned  by  the  Papal  "  Con- 
gregation of  the  Index ; "  and  therefore  Tycho 
might  have  had  as  a  further  motive  a  desire  to 
curry  favour  with  the  authorities  of  the  Church 
of  Rome,  and  to  gratify  his  own  vanity  at  the 
same  time. 

With  these  explanations  it  will  no  longer  be 
misleading  if,  for  convenience  sake,  I  speak  of  a 
certain  great  circle  of  the  heavens  as  apparently 
traversed  by  the  Sun  every  year,  owing  to  the 


74  THE   STORY   OF  THE   SOLAR   SYSTEM. 

revolution  of  our  Earth  round  that  body.  This 
circle  is  called  the  "  Ecliptic,"  and  its  plane  is 
usually  employed  by  astronomers  as  a  fixed  plane 
of  reference.  It  must  be  distinguished  from 'that 
other  great  circle  called  the  "  celestial  equator," 
which  is  the  plane  of  the  Earth's  equator  ex- 
tended towards  the  stars.  The  plane  of  the 
equator  is  inclined  to  the  ecliptic  at  an  angle  of 
about  23^-°,  which  angle  is  known  as  the  "  ob- 
liquity of  the  ecliptic."  It  is  this  inclination 
which  gives  rise  to  the  seasons  which  follow  one 
another  in  succession  during  our  annual  journey 
round  the  Sun.  The  two  points  where  the  ce- 
lestial equator  and  the  ecliptic  intersect  are  called 
the  "  equinoxes,"  of  spring  or  autumn  as  the  case 
may  be ;  the  points  midway  between  these  being 
the  "  solstices,"  of  summer  or  winter  as  the  case 
may  be.  These  words  need  but  little  explana- 
tion, at  any  rate,  as  regards  those  persons  who 
are  able  to  trace  the  Latin  origin  of  the  words. 
<;  Equinox  "  is  simply  the  place  occupied  by  the 
Sun  twice  every  year  (namely  about  March  20 
and  September  22),,  when  day  and  night  are  theo- 
retically equal  throughout  the  world,  when  also 
the  sun  rises  exactly  in  the  east  and  sets  exactly 
in  the  west.  The  "  solstices  "  represent  the  stand- 
ing still  of  the  sun  at  the  given  times  and  places, 
and  are  the  neutral  points  where  the  Sun  attains  its 
greatest  northern  or  southern  declination.  This 
usually  occurs  about  June  21  and  December  21. 
It  must  not  be  forgotten  by  the  way,  that  the 
above  application  of  the  words  "  summer  "  and 
"  winter  "  to  the  solstices  is  only  correct  so  far  as 
concerns  places  in  northern  terrestrial  latitudes — 
Europe  and  the  United  States,  for  instance.  In 
southern  terrestrial  latitudes — for  instance,  when 


THE  T^ARTH.  75 

speaking  of  what  happens  at  the  Cape  of  Good 
Hope  and  in  Australia — the  words  must  be  re- 
versed. 

We  have  seen  in  a  previous  chapter  that 
whilst  the  orbits  of  the  planets  are  nearly  true 
circles,  none  of  them  are  quite  such  :  and  the 
departure  from  the  truly  circular  form  results  in 
some  important  consequences.  Whilst  some  of 
these  are  too  technical  to  be  explained  in  detail 
here,  one  at  least  must  be  referred  to  because 
of  what  it  involves.  Not  only  is  the  Earth's  or- 
bit eccentric  in  form,  but  its  eccentricity  varies 
within  narrow  limits;  and  besides  this  the  orbit 
itself,  as  a  whole,  is  subject  to  a  periodical  shift 
of  place,  from  the  joint  effect  of  all  which  changes 
it  comes  about  that  our  seasons  are  now  of  un- 
equal length,  the  spring  and  summer  quarters  of 
the  year  unitedly  extending  to  186  days,  whilst 
the  autumn  and  winter  quarters  comprise  only 
178  days.  The  sun  therefore  has  the  chance  of 
shining  for  a  longer  absolute  period  of  time  over 
the  northern  hemisphere  than  over  the  southern 
hemisphere  ;  hence  the  northern  is  the  warmer 
of  the  two  hemispheres,  because  it  has  a  better, 
because  a  longer,  chance  of  storing  up  an  accu- 
mulation of  solar  radiant  heat.  Probably  it  is  one 
result  of  this  that  the  north  polar  regions  of  the 
Earth  are  easier  of  access  than  the  south  polar 
regions.  In  the  northern  hemisphere  navigators 
have  reached  to  81°  of  latitude,  whereas  71°  is 
the  highest  limit  yet  attained  in  the  southern 
hemisphere.  Readers  who  have  studied  the  his- 
tory of  explorations  in  the  Arctic  regions  will  not 
need  to  be  reminded  of  the  controversy  which  has 
so  often  arisen  respecting  the  existence  or  non- 
existence  of  an  "  Open  Polar  Sea." 


7 6  THE   STORY   OF   THE   SOLAR   SYSTEM. 

It  has  already  been  hinted  that  it  is  not  an 
easy  matter  to  determine,  when  dealing  with  the 
Earth,  where  astronomy  and  its  allied  sciences, 
geography,  geodesy  and  geology  respectively, 
begin  and  end.  But  as  certain  topics  connected 
with  these  sciences,  such  as  the  rotundity  of  the 
Earth  and  its  rotation  on  its  axis,  will  come  more 
conveniently  under  consideration  in  other  vol- 
umes of  this  series,  I  shall  pass  them  over  and 
only  treat  of  a  few  things  which  more  directly 
concern  the  student  of  nature  observing  either 
with  or  without  the  assistance  of  a  telescope. 

The  fact  that  the  Earth  is  surrounded  by  a 
considerable  atmosphere  largely  composed  of 
aqueous  vapour  has  a  material  bearing  on  the 
success  or  failure  of  observations  made  on  the 
Earth  of  bodies  situated  at  a  distance.  It  may 
be  taken  as  a  general  rule  that  the  nearer  an 
observer  is  to  the  surface  of  the  sea,  or  otherwise 
to  the  surface  of  the  land  at  the  sea-level,  the 
greater  will  be  the  difficulty  which  will  confront 
him  in  carrying  on  astronomical  observations. 
Hence  such  observations  are  generally  made  with 
unsatisfactory  results  on  the  sea  coast  or  on  the 
banks  of  rivers.  An  interesting  but  rather  an- 
cient illustration  of  this  last-named  fact  is  to  be 
found  in  the  circumstance  that  Copernicus,  who 
died  at  the  age  of  70,  complained  in  his  last  mo- 
ments that  much  as  he  had  tried  he  had  never 
succeeded  in  detecting  the  planet  Mercury,  a  fail- 
ure due,  as  Gassendi  supposed,  to  the  vapours 
prevailing  near  the  horizon  at  the  town  of  Thorn 
on  the  banks  of  the  Vistula  where  the  illustrious 
philosopher  lived. 

The  phenomena  depending  on  the  presence  of 
aqueous  vapour  in  the  atmosphere  which  espe- 


THE   EARTH.  77 

cially  come  under  the  notice  of  the  astronomer 
are  Refraction,  Twilight,  and  the  Twinkling  of 
the  Stars. 

Refraction  is  what  it  professes  to  be,  a  bend- 
ing, and  what  is  bent  is  the  ray  of  light  coming 
from  a  celestial  object  to  a  terrestrial  station. 
Olmsted  has  put  the  matter  in  this  way : — "We 
must  consider  that  any  such  object  always  appears 
in  the  direction  in  which  the  last  ray  of  light 
comes  to  the  eye.  If  the  light  which  comes  from 
a  star  were  bent  into  fifty  directions  before  it 
reached  the  eye,  the  star  would  nevertheless  ap- 
pear in  a  line  described  by  the  ray  nearest  the 
eye.  The  operation  of  this-  principle  is  seen  when 
an  oar,  or  any  stick,  is  thrust  into  the  water.  As 
the  rays  of  light  by  which  the  oar  is  seen  have 
their  direction  changed  as  they  pass  out  of  water 
into  air,  the  apparent  direction  in  which  the  body 
is  seen  is  changed  in  the  same  degree,  giving  it  a 
bent  appearance — the  part  below  the  water  hav- 
ing apparently  a  different  direction  from  the  part 
above."  The  direction  of  this  refraction  is  deter- 
mined by  the  general  law  of  optics  that  when  a 
ray  of  light  passes  out  of  a  rarer  into  a  denser 
medium  (for  instance  out  of  air  into  water,  or  out 
of  space  into  the  Earth's  atmosphere)  it  is  bent 
towards  a  perpendicular  to  the  surface  of  the 
medium ;  but  when  it  passes  out  of  a  denser  into 
a  rarer  medium  it  is  bent  / ~rom  the  perpendicular. 
The  effect  of  refraction  is  to  make  a  heavenly 
body  appear  to  have  an  apparent  altitude  greater 
than  its  true  altitude,  so  that,  for  example,  an 
object  situated  actually  in  the  horizon  will  appear 
above  it.  Indeed  it  sometimes  happens  that  ob- 
jects which  are  actually  below  the  horizon  and 
which  otherwise  would  be  invisible  were  it  not 


78  THE  STORY  OF  THE   SOLAR  SYSTEM. 

for  refraction  are  thus  brought  into  sight.  It 
was  in  consequence  of  this  that  on  April  20,  1837, 
the  Moon  rose  eclipsed  before  the  Sun  had  set. 

Sir  Henry  Holland  thus  alludes  to  the  phenom- 
enon : — "  I  am  tempted  to  notice  a  spectacle,  hav- 
ing a  certain  association  with  this  science,  which 
I  do  not  remember  to  have  seen  recorded  either  in 
prose  or  poetry,  though  well  meriting  description 
in  either  way.  This  spectacle  requires,  however, 
a  combination  of  circumstances  rarely  occurring 
— a  perfectly  clear  Eastern  and  Western  horizon, 
and  an  entirely  level  intervening  surface,  such  as 
that  of  the  sea  or  the  African  desert — the  former 
rendering  the  illusion,  if  such  it  may  be  called, 
most  complete  to  the  eye.  The  view  I  seek  to 
describe  embraces  the  orb  of  the  setting  Sun, 
and  that  of  the  full  Moon  rising  in  the  East — 
both  above  the  horizon  at  the  same  time.  The  spec- 
tator on  the  sea  between,  if  he  can  discard  from 
mental  vision  the  vessel  on  which  he  stands,  and 
regard  only  these  two  great  globes  of  Heaven 
and  the  sea-horizon  circling  unbroken  around 
him,  gains  a  conception  through  this  spectacle 
clearer  than  any  other  conjunction  can  give,  of 
those  wonderful  relations  which  it  is  the  triumph 
of  astronomy  to  disclose.  All  objects  are  ex- 
cluded save  the  Sun,  the  Moon,  and  our  own 
Globe  between,  but  these  objects  are  such  in 
themselves  that  their  very  simplicity  and  paucity 
of  number  enhances  the  sense  of  the  sublime. 
Only  twice  or  thrice,  however,  have  I  witnessed 
the  sight  in  its  completeness — once  on  a  Medi- 
terranean voyage  between  Minorca  and  Sardinia 
— once  in  crossing  the  desert  from  Suez  to  Cairo, 
when  the  same  full  Moon  showed  me,  a  few  hours 
later,  the  very  different  but  picturesque  sight  of 


THE  EARTH.  7$ 

one  of  the  annual  caravans  of  Mecca  pilgrims, 
with  a  long  train  of  camels  making  their  night 
march  towards  the  Red  Sea."* 

It  is  due  to  the  same  cause  that  the  Sun  and 
the  Moon  when  very  near  the  horizon  may  often 
be  noticed  to  exhibit  a  distorted  oval  outline. 
The  fact  simply  is,  that  the  upper  and  the  lower 
limbs  undergo  a  different  degree  of  refraction. 
The  lower  limb  being  nearer  the  horizon  is  more 
affected  and-  is  consequently  raised  to  a  greater 
extent  than  the  upper  limb,  the  resulting  effect 
being  that  the  two  limbs  are  seemingly  squeezed 
closer  together  by  the  difference  of  the  two  re- 
fractions. The  vertical  diameter  is  compressed 
and  the  circular  outline  becomes  thereby  an  oval 
outline  with  the  lesser  axis  vertical  and  the 
greater  axis  horizontal. 

Though  the  foregoing  information  merely  em- 
braces a  few  general  principles  and  facts,  the 
reader  will  have  no  difficulty  in  understanding 
that  refraction  exercises  a  very  inconvenient  dis- 
turbing influence  on  observations  which  relate  to 
the  exact  places  of  celestial  objects.  No  such 
observations  are  available  for  mutual  comparison, 
however  great  the  skill  of  the  observer,  or  the 
perfection  of  his  instrument,  unless,  and  until 
certain  corrections  are  applied  to  the  observed 
positions  in  order  to  neutralise  the  disturbing 
effects  of  refraction.  In  practice  this  is  usually 
done  by  means  of  tables  of  corrections,  those  in 
most  general  use  being  Bessel's.  Inasmuch  as  re- 
fraction depends  upon  the  aqueous  vapour  in  the 
atmosphere,  its  amount  at  any  given  moment  is 
affected  by  the  height  of  the  barometer  and  the 

*"  Recollections  of  Past  Life"  2nd  ed.,  p.  305. 
6 


So  THE   STORY   OF  THE  SOLAR   SYSTEM. 

temperature  of  the  air.  Accordingly  when,  for 
any  purpose,  the  utmost  precision  is  required,  it 
is  necessary  to  take  into  account  the  height  of 
the  barometer  and  the  position  of  the  mercury  in 
the  thermometer  at  the  moment  in  question.  At 
the  zenith  there  is  no  refraction  whatever,  objects 
appearing  projected  on  the  background  of  the 
sky  exactly  in  the  position  they  would  occupy 
were  the  earth  altogether  destitute  of  an  atmos- 
phere at  all.  The  amount  of  the  refraction 
increases  gradually,  but  in  accordance  with  a 
very  complex  law,  from  the  zenith  to  the  horizon. 
Thus  the  displacement  due  to  refraction  which  at 
the  zenith  is  nothing  and  at  an  altitude  of  45° 
is  only  57"  becomes  at  the  horizon  more  than  £°. 
One  very  curious  consequence  is  involved  in  the 
fact  that  the  displacement  due  to  refraction  is  at 
the  horizon  what  it  is ;  the  diameter  both  of  the 
Sun  and  Moon  may  be  said  to  be  -J-0,  more  or 
less,  so  that  when  we  see  the  lower  edge  of  either 
of  these  luminaries  just  touching  the  horizon  in 
reality  the  whole  disc  is  completely  below  it,  and 
would  be  altogether  hidden  by  the  convexity  of 
the  earth  were  it  not  for  the  existence  of  the 
earth's  atmosphere  and  the  consequent  refraction 
of  the  rays  of  light  passing  through  it  from  the 
Sun  (or  Moon)  to  the  observer. 

Twilight  is  another  phenomenon  associated 
with  astronomical  principles  and  effects  which  de- 
pends in  some  degree  on  the  Earth's  atmosphere 
and  on  the  laws  which  regulate  the  reflection 
and  refraction  of  light.  After  the  Sun  has  set  it 
continues  to  illuminate  the  clouds  and  upper 
strata  of  the  air  just  as  it  may  often  be  seen 
shining  on  the  tops  of  hills  long  after  it  has  dis- 
appeared from  the  view  of  the  inhabitants  of  the 


THE  EARTH.  8 1 

plains  below,  and  indeed  may  illuminate  the 
chimneys  of  a  house  when  it  is  no  longer  visible 
to  a  person  standing  in  the  garden  below.  The 
air  and  clouds  thus  illuminated  reflect  some  of 
the  Sun's  light  to  the  surface  of  the  earth  lying 
immediately  underneath,  and  thus  produce  after 
sun-set  and  before  sun-rise,  in  a  degree  more  or  less 
considerable  according  as  the  Sun  is  only  a  little 
or  is  much  depressed  below  the  horizon,  that  lumi- 
nous glow  which  we  call  "  twilight.'-  This  word  is 
of  Saxon  origin  and  implies  the  presence  of  a  twin, 
or  double,  light.  As  soon  as  the  Sun  has  disap- 
peared below  the  horizon  all  the  clouds  overhead 
continue  for  a  few  minutes  so  highly  illuminated 
as  to  reflect  scarcely  less  light  than  the  direct 
light  of  the  Sun.  As,  however,  the  Sun  gradu- 
ally sinks  lower  and  lower,  less  and  less  of  the 
visible  atmosphere  receives  any  portion  of  its 
light,  and  consequently  less  and  less  is  reflected 
minute  by  minute  to  the  Earth  at  the  observer's 
station  until  at  length  the  time  comes  when  there 
is  no  sunlight  to  be  reflected — and  it  is  night. 
The  converse  of  all  this  happens  before  and  up 
to  sun-rise;  night  ceases,  twilight  ensues,  gradu- 
ally becoming  more  definite  ;  the  dawn  appears, 
and  finally  the  full  Sun  bursts  forth.  It  may  here 
be  stated  as  a  note  by  the  way  that  the  circum- 
stances under  which  the  Sun  first  shows  itself 
after  it  has  risen  above  the  horizon  has  some 
bearing  on  the  probable  character  of  the  weather 
which  is  at  hand.  When  the  first  indications  of 
day-light  are  seen  above  a  bank  of  clouds  it  is 
thought  to  be  a  sign  of  wind;  but  if  the  first 
streaks  of  light  are  discovered  low  down,  that  is 
in,  or  very  near  the  horizon,  fair  weather  may  be 
expected. 


82  THE  STORY  OF  THE  SOLAR  SYSTEM. 

Twilight  is  usually  reckoned  to  last  until  the 
Sun  has  sunk  18°  below  the  horizon,  but  the 
question  of  its  duration  depends  on  where  the 
observer  is  stationed,^on  the  season  of  the  year, 
and  (in  a  slight  degree)  on  the  condition  of  the 
atmosphere.  The  general  rule  is  that  the  twilight 
is  least  in  the  tropics  and  increases  as  the  ob- 
server moves  away  from  the  equator  towards 
either  pole.  Whilst  in  the  tropics  a  depression 
of  16°  or  17°  is  sufficient  to  put  an  end  to  the 
phenomenon,  in  the  latitude  of  England  a  de- 
pression of  from  17°  to  20°  is  required.  As  im- 
plied above,  it  varies  with  the  latitude  ;  and  as 
regards  the  different  seasons  of  the  year,  it  is 
least  on  March  i  and  October  12,  being  three 
weeks  before  the  vernal  equinox  and  three  weeks 
after  the  autumnal  equinox.  The  duration  at 
the  equator  may  be  about  i  hour  12  minutes; 
it  amounts  to  nearly  2  hours  at  the  latitude  of 
Greenwich,  and  so  on  towards  the  pole.  At 
each  pole  in  turn  the  Sun  is  below  the  horizon 
for  6  months,  but  as  it  is  less  than  18°  below 
the  horizon  for  about  3^-  of  those  6  months  it 
may  be  said  that  there  is  a  continual  twilight  for 
those  3-J-  months.  Something  of  the  same  sort 
of  thing  as  this  occurs  in  the  latitude  of  Green- 
wich, for  there  is  no  true  night  at  Greenwich 
from  May  22  to  July  21,  but  constant  twilight 
from  sunset  to  sunrise,  or  2  months  of  twilight 
in  all.  Though  twilight  at  the  equator  is  com- 
monly set  down  as  lasting  about  an  hour,  this 
period  is  there,  as  elsewhere,  affected  by  the 
elevation  of  the  observer  above  the  sea-level. 
Where  the  air  is  very  rarified,  as  at  places  situ- 
ated as  Quito  and  Lima  are,  the  twilight  is  said 
to  last  no  more  than  20  minutes,  and  this  would 


THE   EARTH.  83 

accord  with  the  theory  that  where  there  is  no  air 
at  all  (e.  g.,  on  the  Moon)  there  is  no  twilight  at 
all.  The  greater  purity  and  clearness  of  moun- 
tain air,  rarified  as  it  is,  is  another  cause  which 
contributes  to  vary  by  reducing  the  duration  of 
twilight. 

It  is  sometimes  stated  that  a  secondary  twilight 
may  be  noticed,  and  Sir  John  Herschel  has  spoken 
of  it  as  "consequent  on  a  re-reflection  of  the  rays 
dispersed  through  the  atmosphere  in  the  primary 
one.  The  phenomenon  seen  in  the  clear  atmos- 
phere of  the  Nubian  Desert,  described  by  travel- 
lers under  the  name  of  the  *  afterglow/  would 
seem  to  arise  from  this  cause."  I  am  not  ac- 
quainted with  any  records  which  throw  light  on 
these  remarks  of  Sir  John  Herschel. 

The  phenomenon  of  twinkling  is  a  subject 
which  has  been  much  neglected,  possibly  on  ac- 
count of  its  apparent,  but  only  apparent,  sim- 
plicity. The  familiar  verse  of  our  days  of  child- 
hood— 

*'  Twinkle,  twinkle  little  star, 

How  I  wonder  what  you  are, 

Up  above  the  earth  so  high, 

Like  a  diamond  in  the  sky," 

contains  even  in  this  simple  form  a  good  deal  of 
food  for  reflection ;  whilst  the  new  version — 

"  Twinkle,  twinkle  little  star, 
Now  we've  found  out  what  you  are, 
When  unto  the  midnight  sky 
We  the  spectroscope  apply," 

does  so  yet  more. 

As  an  optical  phenomenon  the  twinkling,  or  to 
use  the  more  scientific  phrase,  the  scintillation, 
of  the  stars  is  a  matter  which  ha.s  been  strangely 


84  THE  STORY  OF  THE  SOLAR  SYSTEM. 

ignored  by  physicists.  Indeed,  the  only  investi- 
gators who  seem  to  have  dealt  with  it  in  any  sort 
of  detail  are  two  Italians,  Secchi  and  Respighi, 
Dufour,  a  Frenchman,  Montigny,  a  Belgian,  and 
the  Rev.  E.  Ledger,  an  Englishman.  Secchi  has 
truly  remarked  that  the  twinkling  of  the  stars  is 
one  of  the  most  beautiful  of  the  minor  phenomena 
of  the  heavens.  Light,  sometimes  bright,  some- 
times feeble,  sometimes  white,  sometimes  red, 
darts  about  in  intermittent  gleams,  like  the  spark- 
ling flashes  of  a  well-cut  diamond,  and  works  upon 
the  feelings  of  even  the  most  stolid  spectator. 
The  theory  of  twinkling  is  still  surrounded  by 
many  difficulties.  One  thing,  however,  is  certain 
— it  has  nothing  to  do  with  recurrent  changes  in 
the  intrinsic  light  or  physical  condition  of  the 
star  itself,  but  arises  during  the  passage  of  its 
rays  through  our  atmosphere  ;  it  depends,  there- 
fore, in  some  way  or  other  on  the  varying  con- 
ditions of  the  atmosphere.  On  the  summit  of 
high  mountains,  according  to  the  observations 
of  all  careful  observers  (notably  Tacchini,  who 
studied  the  subject  on  Mount  Etna),  the  light  of 
the  stars  is  steady,  like  that  of  the  planets  ;  and 
it  is  so  likewise  during  the  hours  of  calm  which 
often  precede  terrestrial  storms.  The  vibrations 
are  usually  more  frequent  near  the  horizon,  and 
diminish  with  the  elevation  of  the  star  above  the 
horizon ;  in  other  words,  with  the  lessening  of 
the  thickness  of  the  atmospheric  strata  which 
the  rays  of  light  have  to  traverse.  Nevertheless, 
during  windy  weather,  and  specially  with  north- 
erly wind,  it  may  be  noticed  that  the  stars 
twinkle  high  up  above  the  horizon,  and  even  as 
far  as  the  zenith.  From  these  and  other  similar 
considerations  we  are  justified  in  drawing  the 


THE   EARTH.  85 

conclusion  that  twinkling  largely  depends  on  the 
condition  and  movements  of  the  atmosphere. 

Secchi  further  points  out  that  it  is  impossible 
to  study  carefully  with  the  naked  eye  all  the 
features  of  twinkling,  and  that  telescopic  assist- 
ance is  imperatively  necessary.  When,  with  the 
aid  of  a  telescope,  we  scrutinise  a  star  during  a 
disturbed  evening  marked  by  much  twinkling  we 
see  an  image  diffused  and  undefined  and  sur- 
rounded by  rays,  as  if  several  images  were  super- 
posed, and  were  jumping  about  rapidly.  On  such 
occasions  we  do  not  see  that  little  defined  disc 
surrounded  by  motionless  diffraction  rings,  ordi- 
narily indicative  of  a  tranquil  atmosphere.  With 
a  telescope  armed  with  a  medium  power,  the  field 
of  view  of  which  is  more  extensive  than  that  of  a 
high  power,  we  find  that  if  a  light  tap  is  given 
to  the  telescope,  the  ordinary  simple  image  is 
changed  into  a  luminous  curve,  the  perimeter  of 
which  is  formed  entirely  of  a  succession  of  arcs 
exhibiting  the  colours  of  the  rainbow.  This  col- 
oured curve  does  not,  in  principle,  differ  from 
what  one  sees  on  swinging  round  and  round  in 
the  air  such  a  thing  as  a  stick,  the  end  of  which 
is  alight,  having  been  freshly  taken  from  a  fire. 
The  glowing  tip  produces  in  appearance  a  contin- 
uous arc,  the  result  of  the  persistence  of  the  im- 
age of  the  tip  on  the  retina.  In  such  a  case  the 
colour  is  constant,  because  the  illumination  re- 
sulting from  the  blazing  wood  does  not  vary ;  but 
in  the  case  of  a  star  the  arcs  are  differently  col- 
oured during  the  very  brief  space  of  time  in  which 
the  vibrating  telescope  transports  the  image  from 
one  side  to  another  of  the  visible  field.  This  ex- 
periment is  from  its  nature  very  crude,  but  the 
idea  was  improved  upon  and  reduced  to  a  syste- 


86  THE  STORY  OF  THE   SOLAR  SYSTEM. 

matic  shape  by  Montigny,  who  introduced  into 
his  telescope,  at  a  certain  distance  from  the  eye- 
piece, a  concave  lens  eccentrically  placed  with 
respect  to  the  axis  of  the  instrument,  and  endued 
with  a  rapid  movement  of  rotation  imparted  by 
suitable  mechanism.  He  thus  obtained  images 
which  revolved  with  regularity,  and  so  was  able 
to  submit  certain  features  of  the  phenomenon  to 
a  definite  system  of  measurement.  To  cut  a  long 
story  short,  Montigny  started  with  the  assumption 
(made  good  by  the  sequel)  that  possibly  stars 
were  affected  in  their  twinkling  by  intrinsic  con- 
stitutional differences;  and  that  possibly  Secchi's 
classification  of  stars  into  four  types  (a  classifica- 
tion which  depends  on  the  spectra  which  they 
yield)  might  put  him  on  the  track  of  some  intelli- 
gible conclusions  with  respect  to  the  theory  of 
twinkling.* 

The  results  he  ultimately  arrived  at  were,  that 
the  yellow  and  red  stars  of  the  Ilnd  and  Illrd 
types  twinkle  less  rapidly  than  the  white  stars 
of  the  1st  type.  Whilst  the  average  number  of 
scintillations  per  second  of  the  stars  of  type  III. 
were  56,  those  of  type  II.  were  69,  and  those  of 
type  I.  86.  These  differences  may  be  confidently 
said  to  depend  upon  too  many  observations  of 
too  many  different  stars  to  be  fortuitous.  Mon- 
tigny also  arrived  at  a  number  of  incidental  con- 
clusions of  considerable  interest.  The  one  main 
thread  running  through  them,  is  that  there  is  a 
connection  between  the  twinkling  of  a  star  and 
its  spectrum,  which  had  never  before  been  thought 
of.  We  are  justified,  indeed,  in  going  so  far  as  to 

*  For  some  information  respecting  these  Secchi  "  Types  " 
of  Stars,  see  my  "  Story  of  the  Stars"  2nd  ed.,  p.  140. 


THE  EARTH.  87 

say,  that  Montigny's  observations  point  distinctly 
to  a  law  on  this  subject,  the  law  being  that  the 
more  the  spectrum  of  a  star  is  interrupted  by  dark 
lines,  the  less  frequent  are  its  scintillations.  The 
individual  character  of  the  light,  therefore,  emitted 
by  any  given  star  appears  to  affect  its  twinkling, 
both  as  regards  the  frequency  thereof  and  the  col- 
ours displayed. 

Montigny  collected  some  other  interesting 
facts  with  reference  to  twinkling,  which  may  here 
be  stated  in  a  concise  form.  There  is  a  greater 
display  of  twinkling  in  showery  weather,  than  when 
the  atmosphere  is  in  a  normal  condition ;  and  in 
winter  than  in  summer,  whatever  may  be  the 
weather.  In  dry  weather  in  Spring  and  Autumn 
the  twinkling  is  about  the  same,  but  wet  has 
more  effect  in  Autumn  than  in  Spring  in  develop- 
ing the  phenomenon.  Variations  in  the  baro- 
metric pressure  and  in  the  humidity  of  the  air 
also  affect  the  amount  of  twinkling ;  there  is 
more  before  a  rainy  period,  likely  to  last  2  or  3 
days,  than  before  a  single,  or,  so  to  speak,  casual 
rainy  day.  Twinkling  also  varies  with  the  aggre- 
gate total  rain-fall  of  any  group  of  days,  being 
more  pronounced  as  the  rain-fall  is  greater,  but 
decreasing  suddenly  and  considerably  as  soon  as 
the  rainy  condition  of  the  atmosphere  has  passed 
away.  The  number  of  scintillations  found  to  be 
observable  with  the  aid  of  Montigny's  instrument 
(which  he  called  a  "  scintillometre  "),  varied  from  a 
minimum  of  50  during  June  and  July,  to  97  in 
January,  and  101  in  February,  increasing  and 
decreasing  in  regular  sequence  from  month  to 
month.  When  an  Aurora  Borealis  is  visible,  there 
is  a  marked  increase  in  the  amount  of  twinkling. 
It  would  be  interesting  to  follow  up  this  last 


88  THE  STORY  OF   THE  SOLAR  SYSTEM. 

named  discovery  by  an  endeavour  to  ascertain 
whether  the  fluctuations  which  are  coincident  in 
point  of  time  with  an  Auroral  display  depend 
upon  optical  considerations  connected  with  the 
Aurora,  or  on  physical  considerations  having  any 
relation  to  the  increased  development  of  terres- 
trial magnetism. 

I  have  been  thus  particular  in  unfolding  some- 
what fully  the  present  state  of  our  knowledge 
concerning  the  twinkling  of  the  stars,  because  it 
is  evident  that  there  are  many  interesting,  points 
connected  with  it,  which  may  be  studied  by  any 
patient  and  attentive  star-gazer,  and  which  do  not 
need  the  instrumental  appliances  and  technical  re- 
finements which  are  only  to  be  found  in  fully- 
equipped  public  and  private  observatories. 

It  should  be  mentioned  in  conclusion  that  the 
planets  twinkle  very  little,  or,  more  often,  not  at 
all.  This  is  mainly  due  to  the  fact  that  they  ex- 
hibit discs  of  sensible  diameter  and  therefore  that 
there  is,  as  Young  puts  it,  u  a  general  unchanging 
average  of  brightness  for  the  sum  total  of  all  the 
luminous  points  of  which  the  disc  is  composed. 
When,  for  instance,  point  A  of  the  disc  becomes 
dark  for  a  moment,  point  B,  very  near  to  it,  is 
just  as  likely  to  become  bright;  the  interference 
conditions  being  different  for  the  2  points.  The 
different  points  of  the  disc  do  not  keep  step,  so  to 
speak,  in  their  twinkling/'  The  non-twinkling  of 
planets  because  they  possess  sensible  discs  is 
often  available  as  a  means  for  determining  when 
a  planet  is  looked  for,  which,  of  several  objects 
looked  at,  is  the  planet  wanted  and  which  are 
merely  stars, 


THE  MOON.  89 


CHAPTER   VI. 

THE    MOON. 

THE  Moon  being  merely  the  satellite  of  a 
planet,  to  wit,  the  Earth,  it  should,  according  to 
the  plan  of  this  book,  be  included  in  the  chapter 
which  deals  with  its  primary;  but  for  us  inhabit- 
ants of  the  Earth  the  Moon  has  so  many  special 
features  of  interest  that  it  will  be  better  to  give 
it  a  special  chapter  to  itself. 

We  may  regard  the  Moon  in  a  twofold  aspect, 
and  consider  what  it  is  as  a  mere  object  to  look 
at,  and  what  it  does  for  us ;  probably  my  present 
readers  will  prefer  that  most  prominence  shall  be 
given  to  the  former  aspect.  The  Moon  as  seen 
with  the  naked  eye  exhibits  a  silvery  mass  of  light, 
which  at  the  epoch  of  what  is  called  u  full  Moon  " 
has  a  seemingly  even  circular  outline.  Full  or  not 
full,  its  surface  appears  to  be  irregularly  shaded 
or  mottled.  The  immediate  cause  of  this  shading 
is  the  fact  that  the  surface  of  the  Moon,  not  being 
really  smooth,  reflects  irregularly  the  Sun's  light 
which  falls  upon  it.  The  causa  causa/is  of  this  is 
the  existence  of  numerous  mountains  and  valleys 
on  its  surface,  and  which  were  first  discovered  to 
be  such  by  Galileo.  That  there  are  mountains  is 
prov-ed  by  the  shadows  cast  by  their  peaks  on  the 
surrounding  plains,  when  the  Sun  illuminates  the 
Moon  obliquely — that  is,  when  the  Moon  is  shin- 
ing either  as  a  crescent  or  gibbous.  Such  shadows, 
however,  disappear  at  the  phase  of  "  Full-Moon," 
because  the  Sun's  rays  then  fall  perpendicularly 
on  the  Moon's  surface.  When  the  Moon  presents 
either  a  crescent  or  a  gibbous  form  (in  point  of 


9° 


THE  STORY  OF  THE   SOLAR   SYSTEM. 


fact  when  it  presents  any  form  except  that  of 
"  Full-Moon  "),  the  boundary  line  which  separates 
the  illuminated  from  the  unilluminated  portion 

(and  which  boun- 
dary line  is  gen- 
erally spoken  of 
as  the  u  termina- 
tor") has  a  rough, 
jagged  appear- 
ance ;  this  is  due 
to  the  fact  that 
the  Sun's  light 
falls  first  on  the 
summits  of  the 
peaks,  and  that 
the  adjacent  val- 
leys and  declivi- 
FIG.  n.-Mare  Crisium.  ties  are  in  shade. 

(Lick  Observatory  photographs.)  These   remain    SO 

till  by  reason  of 

the  Moon's  progress  in  its  orbit  a  sufficient  time 
has  elapsed  for  the  Sun  to  penetrate  to  the  bot- 
tom of  the  valleys.  With  this  explanation  the 
reader  will  have  no  difficulty  in  realising  why  the 
terminator  always  exhibits  an  irregular  or  jagged 
edge. 

Various  mountains  on  the  Moon  to  the  num- 
ber of  more  than  a  thousand  have  been  mapped, 
and  their  elevations  calculated.  Of  these  fully 
half  have  received  names,  being  those  of  men  of 
various  dates  and  nationalities,  who  have  figured 
conspicuously  in  the  annals  of  science,  including 
some,  however,  who  have  not  done  so.  Whilst 
many  of  these  mountains  are  isolated  elevations, 
not  a  few  form  definite  chains  of  mountains,  and 
to  certain  of  these  chains  definite  names,  bpr- 


THE  MOON.  91 

rowed  from  the  Earth,  have  been  given.  Thus 
we  find  on  maps  of  the  Moon  the  "  Apennines," 
the  "Alps,"  the  "Altai  Mountains,"  the  "  Dorfel 
Mountains,"  the  "  Caucasus  Mountains,"  and 
so  on. 

Besides  the  mountains  there  exist  on  the  Moon 
a  number  of  plains  analogous  in  some  sense  to 
the  "  steppes "  of  Asia  and  the  "  prairies "  of 
North  America.  These  were  termed  "  seas  "  in 
the  early  days  of  the  telescope,  because  it  was 
assumed  that  as  they  were  so  large  and  so  smooth 
they  were  vast  tracts  of  water.  This  supposition 
has  long  ago  been  overthrown,  but  the  names 
have  been  retained  as  a  matter  of  convenience. 
Hence  it  comes  about  that  in  descriptions  of  the 
Moon  one  meets  with  such  names  as  Mare  Imbrium, 
the  "  Sea  of  Showers  "  ;  Mare  Serenitatis,  the  "  Sea 
of  Serenity  " ;  Mare  Tranquillitatis,  the  "  Sea  of 
Tranquillity  "  ;  and  so  on.  It  seems  probable 
that  the  so-called  seas  represent  in  nearly  its 
original  form  what  was  once  the  original  surface 
of  the  Moon  before  the  mountains  were  formed. 
A  confirmation  of  this  idea  is  to  be  found  in  the 
fact  that  though  these  plains  are  fairly  level  sur- 
faces compared  with  the  masses  of  mountains 
which  hedge  them  in  on  all  sides,  yet  the  plains 
themselves  are  dotted  over  with  inequalities  (small 
elevations  and  pits),  which  seem  to  suggest  that 
some  of  them  might  eventually  have  developed 
into  mountains  if  the  further  formation  of  moun- 
tains had  not  been  arrested  by  the  fiat  of  the 
Creator. 

Though  hitherto  we  have  been  speaking  of 
the  mountains  of  the  Moon  under  that  generic 
title,  it  is  necessary  for  the  reader  to  understand 
that  the  Moon's  surface  exhibits  everywhere  re- 


92  THE  STORY  OF  THE  SOLAR  SYSTEM. 

markable  illustrations  of  those  geological  pro- 
cesses which  we  on  the  earth  associate  with  the 
word  "  volcano."  There  cannot  be  the  least  doubt 
that  the  existing~surface  of  the  Moon,  as  we  see 
it,  owes  all  its  striking  features  to  volcanic  ac- 
tion, differing  little  from  the  volcanic  action  to 
which  we  are  accustomed  on  the  earth.  That 
this  theory  is  well  founded  may  be  very  easily 
inferred  by  comparing  the  structural  details  of 
certain  terrestrial  volcanoes  and  their  surround- 
ings with  a  typical  lunar  mountain,  or  indeed,  I 
might  say,  with  any  lunar  mountain.  This  point 
was  very  well  worked  out  some  40  years  ago  by 
Professor  Piazzi  Smyth,  who  placed  on  pictorial 
record  his  results  of  an  examination  and  survey 
of  the  Peak  of  Teneriffe.  Any  person  seeing 
side  by  side  one  of  Smyth's  pictures  of  Teneriffe 
and  a  picture  of  any  average  lunar  crater  would 
find  great  difficulty  if  the  pictures  were  not  label- 
led in  determining  which  was  which. 

The  one  special  feature  of  the  Moon,  which 
never  fails  to  attract  the  attention  of  everybody 
who  looks  at  our  satellite  for  the  first  time 
through  a  telescope,  are  the  crater  mountains, 
which  indeed  constitute  an  immense  majority  of 
all  the  lunar  mountains.  Their  outline  almost 
always  conforms,  more  or  less,  to  that  of  the 
circle,  but  when  seen  near  either  limb  of  the 
Moon  they  often  appear  considerably  oval  simply 
because  they  are  then  seen  considerably  fore- 
shortened. In  their  normal  form  they  exhibit  a 
basin  bounded  by  a  ridge,  with  a  conical  eleva- 
tion in  the  centre  of  the  basin,  the  basin  and  the 
cone  together  being  evidently  the  result  of  an 
uprush  of  gases  breaking  through  the  outer  crust 
of  the  Moon  and  carrying  with  them  masses  of 


THE   MOON.  93 

molten  lava.  This  lava,  with  perhaps  the  ma- 
terials in  fragments,  projected  in  the  first  instance 
up  into  the  air,  fell  back  on  to  the  Moon  forming- 
first  of  all  the  outer  edge  of  the  basin,  and  sub- 
sequently, as  the  eruptive  force  became  weakened, 
the  small  central  accumulation,  which  took,  as  it 
naturally  would  do,  a  conical  shape.  An  experi- 
mental imitation  of  the  process  thus  inferred  was 
carried  out  some  years  ago  by  a  French  physicist, 
Bergeron,  who  acted  upon  a  very  fusible  mixture 
of  metals  known  as  Wood's  alloy  by  forcing 
through  it  a  current  of  hot  air.  The  success  of 
this  experiment  was  complete,  and  Bergeron  con- 
sidered that  his  experiments,  taken  as  a  whole, 
were  calculated  to  throw  much  light  on  the  past 
history  of  the  Moon. 

Several  observers  at  various  times  have  fan- 
cied they  have  seen  signs  that  the  lunar  mountain 
Aristarchus  was  an  active  volcano  even  up  to  the 
present  century ;  but  it  admits  of  no  doubt  that 
this  idea  is  altogether  a  misconception,  and  that 
what  they  saw  as  a  faint  illumination  of  the  sum- 
mit of  Aristarchus  was  no  more  than  an  effect  of 
earth-shine.  On  the  general  question  of  volcanic 
action  on  the  Moon,  Sir  John  Herschel  summed 
up  as  follows:  — "  Decisive  marks  of  volcanic 
stratification  arising  from  successive  deposits  of 
ejected  matter,  and  evident  indications  of  lava 
currents,  streaming  outwards  in  all  directions, 
may  be  clearly  traced  with  powerful  telescopes. 
In  Lord  Rosse's  magnificent  Reflector  the  flat 
bottom  of  the  crater  called  Albategnius  is  seen  to 
be  strewed  with  blocks  not  visible  in  inferior 
telescopes,  while  the  exterior  ridge  of  another 
(Aristillus)  is  all  hatched  over  with  deep  gulleys 
radiating  towards  its  centre/' 


94  THE  STORY  OF  THE  SOLAR  SYSTEM. 

The  valleys  and  clefts  or  rills  visible  on  the 
Moon's  surface  constitute  another  remarkable 
feature  in  the  topography  of  our  satellite.  The 
valleys,  properly  so-called,  require  no  particular 
comment,  because  they  are  just  what  their  name 
implies — hollows  often  many  miles  long  and 
several  miles  wide.  The  clefts  or  rills,  however, 
are  more  mysterious,  by  reason  of  their  great 
length  and  remarkable  narrowness.  One  is  al- 
most led  to  infer  that  they  are  naught  else  but 
cracks  in  the  lunar  crust,  the  result  of,  sudden 
cooling,  how  caused  is  of  course  not  known. 

There  is  another  lunar  feature  to  be  mentioned 
somewhat  akin  to  the  foregoing  in  appearance 
but  apparently,  however,  owing  its  origin  to  a 
different  cause.  I  refer  to  the  systems  of  bright 
streaks  which,  especially  at  or  near  the  time  of 
full  Moon,  are  seen  to  radiate  from  several  of 
the  largest  craters,  and  in  particular  from  Tycho, 
Copernicus,  Kepler  and  Aristarchus.  These  bright 
streaks  extend  in  many  cases  far  beyond  what 
may  fairly  be  considered  as  the  neighbourhood 
of  the  craters  from  which  they  start,  traversing 
distant  mountains,  valleys  and  other  craters  in  a 
way  which  renders  it  very  difficult  to  assign  an 
explanation  of  their  origin. 

There  are  13  areas  on  the  Moon,  which  used 
to  be  regarded  as  "  seas,"  one  of  them,  however, 
bearing  the  name  of  "  Oceanus  Procellarum"  the 
"  Ocean  of  Storms  "  ;  but  besides  these  there  are 
several  bays,  termed  in  Latin  Sinus,  of  which  the 
most  important  is  the  Sinus  Iridum  or  the  "  Bay 
of  Rainbows,"  a  beautiful  spot  on  the  northern 
border  of  the  Mare  Imbrium,  and  best  seen  when 
the  Moon  is  between  9  and  10  days  old.  The 
summits  of  the  semi-circular  range  of  rocks 


THE  MOON.  95 

which  enclose  the  bay  are  then  strongly  illumi- 
nated and  a  greenish  shadow  marks  the  valley  at 
its  base.  By  the  way,  it  is  worth  mentioning 
that  not  a  few  of  the  lunar  seas,  so-called,  seem 
to  be  pervaded  by  a  greenish  hue,  though  no 
particular  explanation  of  this  fact  is  forthcoming. 

Much  controversy  has  ranged  round  the  ques- 
tion whether  or  not  the  Moon  has  an  atmosphere. 
Without  doubt  the  preponderance  of  opinion  is 
on  the  negative  side  though  it  must  be  admitted 
that  some  observers  of  eminence  have  suggested 
that  there  are  indeed  traces  of  an  atmosphere  to 
be  had,  but  that  it  is  extremely  attenuated  and 
of  no  great  extent,  otherwise  it  must  render 
its  presence  discoverable  by  optical  phenomena 
which  it  is  certain  cannot  be  detected. 

A  brief  reference  may  here  be  made  to  a  curi- 
ous phenomenon  sometimes  seen  in  connection 
with  occultations  of  stars  by  the  Moon.  Premis- 
ing that  an  "  occultation  "  is  the  disappearance  of 
a  star  behind  the  solid  body  of  the  Moon  by  rea- 
son of  the  forward  movement  of  the  Moon  in  her 
orbit,  it  must  be  stated  that  though  generally  the 
Moon  extinguishes  the  star's  light  instantane- 
ously, yet  this  does  not  invariably  happen,  for 
sometimes  the  star  seems  to  hang  upon  the 
Moon's  limb  as  if  reluctant  to  disappear.  No 
very  clear  or  satisfactory  explanation  of  this 
phenomenon  has  yet  been  given ;  the  existence 
of  a  lunar  atmosphere  would  be  an  explanation, 
and  accordingly  this  anomalous  appearance,  seen 
on  occasions,  has  been  advanced  in  support  of 
the  theory  that  a  lunar  atmosphere  does  exist ; 
but,  nevertheless,  astronomers  do  not  accept  that 
idea. 

Any  one  desirous  of  carrying  out  a  careful 
7 


96  THE   STORY  OF  THE   SOLAR   SYSTEM. 

study  of  the  Moon's  surface  must  be  provided 
with  a  good  map,  and  for  general  purposes  none 
is  so  convenient  or  accessible  as  Webb's,  reduced 
from  Beer  and  Madler's  Mappa  Selenographica 
published  in  1837,  of  which  another  reproduction 
is  given  in  Lardner's  Astronomy.  Those,  however, 
who  would  desire  to  study  the  Moon  with  the  ut- 
most attention  to  detail  must  provide  themselves 
with  Schmidt's  map  published  in  1878  at  the 
expense  of  the  German  Government.  When  it  is 
stated  that  this  map  represents  the  Moon  on  a 
circle  y-J-  feet  in  diameter,  the  size  and  amount  of 
detail  in  it  will  be  readily  understood.  Special 
books  on  the  Moon  furnishing  numerous  engrav- 
ings and  detailed  descriptions  have  been  written 
by  Carpenter  and  Nasmyth  (jointly)  and  by 
Neison. 

Various  attempts  have  been  made  to  deter- 
mine the  amount  of  light  reflected  by  the  Moon, 
and  also  the  question  whether  it  yields  any  meas- 
urable amount  of  heat.  As  regards  the  light  of 
the  full  Moon  compared  with  that  of  the  Sun,  the 
estimates  range  from  TnnAnnr  to  -gTnjW*  a  discrep- 
ancy not  perhaps  greater  than  might  be  expected 
under  the  circumstances  of  the  case. 

With  respect  to  the  heat  possessed  by,  or 
radiated  from  the  Moon's  surface,  the  conclu- 
sions of  those  who  have  attempted  to  deal  with 
the  matter  are  less  consistent.  As  regards  the 
surface  of  the  Moon  itself  Sir  John  Herschel  was 
of  opinion  that  it  is  heated  at  least  to  the  tem- 
perature of  boiling  water,  but  that  owing  to  the 
radiant  heat  having  to  pass  through  our  atmos- 
phere, which  acts  as  an  obstacle,  it  is  no  wonder 
that  it  should  be  difficult  for  us  to  become  con- 
scious of  its  existence.  In  1846  Melloni,  by  con- 


THE   MOON.  97 

centrating  the  rays  of  the  Moon  with  a  lens  3 
feet  in  diameter,  thought  he  detected  a  sensible 
elevation  of  temperature;  and  in  1856  C.  P. 
Smyth  at  Teneriffe,  but  with  inferior  instru- 
mental appliances,  arrived  at  the  same  conclu- 
sion. Though  Professor  Tyndall  in  1861  obtained 
a  contrary  result,  yet  the  most  recent  experi- 
ments by  the  younger  Earl  of  Rosse,  Professor 
Langley,  and  others,  all  tend  to  show  that  the  Moon 
does  really  radiate  a  certain  infinitesimally  small 
amount  of  heat.  Perhaps,  however,  it  will  be 
best  to  give  Langley's  ideas  as  to  this  in  his  own 
words  : — "  While  we  have  found  abundant  evi- 
dence of  heat  from  the  Moon,  every  method  we 
have  tried,  or  that  has  been  tried  by  others,  for 
determining  the  character  of  this  heat  appears  to 
us  inconclusive ;  and  without  questioning  that 
'the  Moon  radiates  heat  earthward  from  its  soil, 
we  have  not  yet  found  any  experimental  means 
of  discriminating  with  such  certainty  between 
this  and  reflected  heat  that  it  is  not  open  to  mis- 
interpretation." It  is  obvious  from  the  foregoing 
that  we  on  the  Earth  need  not  concern  ourselves 
very  much  about  lunar  heat ;  and  I  will  only  add 
that  F.  W.  Very,  by  an  ingenious  endeavour  to 
localise  the  Moon's  radiant  heat,  has  been  able, 
he  thinks,  to  establish  the  fact  that  on  the  part 
of  the  Moon  to  which  the  Sun  is  setting,  what  he 
calls  the  heat-gradient  (using  a  phrase  suggested 
by  terrestrial  meteorology)  appears  to  be  steeper 
than  on  that  part  to  which  the  Sun  is  rising.  Gen- 
erally, Very's  observations  accord  fairly  with  Lord 
Rosse's. 

The  Moon  revolves  round  the  Earth  in  27  d. 
7  h.  43  m.  ii  s.  at  a  mean  distance  of  237,300 
miles,  in  an  orbit  which  is  somewhat,  but  not 


98  THE  STORY  OF  THE   SOLAR  SYSTEM. 

very,  eccentric.  Its  angular  diameter  at  mean 
distance  is  31'  5",  or,  say,  just  over  -J-0.  The  real 
diameter  may  be  called  2160  miles. 

A  few  words  will  probably  be  expected  by  the 
reader  on  the  subject  of  lunar  influences  on  the 
weather,  and  generally  ;  this  being  a  matter  highly 
attractive  to  the  popular  mind.  The  truth  ap- 
pears to  lie,  as  usual,  between  two  extremes  of 
thought.  The  Moon,  of  course,  is  the  main  cause 
of  the  tides  of  the  Ocean,  and  it  is  not  entirely 
inconceivable  that  tidal  changes  imparted  to  vast 
masses  of  water  may  be  either  synchronous  with, 
or  may  in  some  way  engender,  analogous  move- 
ments in  the  Earth's  atmosphere;  though  no  dis- 
tinct proofs  of  this,  as  a  determinate  fact,  can  be 
brought  forward. 

There  is  no  doubt  whatever  that  at  or  near 
the  time  of  full  Moon,  evening  clouds  tend  to  dis-« 
perse  as  the  Moon  comes  up  to  the  meridian, 
and  that  by  the  time  the  Moon  has  reached  the 
meridian  a  sky  previously  overcast  will  have  be- 
come almost  or  quite  clear.  Sir  John  Herschel 
has  alluded  to  this  by  speaking  of  a  "  tendency  to 
disappearance  of  clouds  under  a  full  Moon"; 
and  he  considers  this  "  fully  entitled  to  rank  as  a 
meteorological  fact."  He  goes  on,  not  unnatu- 
rally, to  suggest  the  obvious  thought  that  such 
dissipation  of  terrestrial  clouds  is  due  to  the  cir- 
cumstance that,  assuming  heat  really  comes  by 
radiation  from  the  Moon  (and  we  have  seen  on  a 
previous  page  the  probability  of  this)  such  radi- 
ant heat  will  be  more  potential  if  it  falls  on  the 
Earth  perpendicularly,  as  from  a  Meridian  Moon, 
than  if  it  comes  to  us  at  any  one  locality  from  a 
Moon  low  down  in  the  observer's  horizon,  and 
therefore  has  to  pass  through  the  denser  strata 


THE   MOON.  99 

of  the  Earth's  atmosphere  and  surfer  material 
enfeeblement  accordingly.  I  am  aware  that  Mr. 
Ellis,  late  of  the  Royal  Observatory,  Greenwich, 
has  sought  to  show  by  a  seemingly  powerful 
array  of  statistics  that  the  idea  now  under  con- 
sideration is  unfounded, -but  I  consider  that  we 
have  here  only  one  more  illustration  of  the  fa- 
miliar statement  that  you  can  prove  anything 
you  like  by  statistics.  I  am  firmly  convinced,  as 
the  result  of  more  than  30  years'  observation, 
that  terrestrial  clouds  do  disperse  under  the  cir- 
cumstances stated.  Sir  J.  Herschel  added  that 
his  statement  proceeded  from  his  own  observa- 
tion "  made  quite  independently  of  any  knowledge 
of  such  a  tendency  having  been  observed  by 
others.  Humboldt,  however,  in  his  Personal  Nar- 
rative, speaks  of  it  as  well  known  to  the  pilots 
and  seamen  of  Spanish  America."  Sir  John  Her- 
schel further  remarked  : — "  Arago  has  shown  from 
a  comparison  of  rain,  registered  as  having  fallen 
during  a  long  period,  that  a  slight  preponderance 
in  respect  of  quantity  falls  near  the  *  new '  Moon 
over  that  which  falls  near  the  '  full.'  This  would 
be  a  natural  and  necessary  consequence  of  a  pre- 
ponderance of  a  cloudless  sky  about  the  '  full,' 
and  forms,  therefore,  part  and  parcel  of  the  same 
meteorological  fact." 

Bernadin  has  asserted  it  to  be  a  fact  that  many 
thunderstorms  occur  about  the  period  of  "  new  " 
or  "  full  "  Moon.  But  what  I  want  most  to  warn 
the  reader  against  is  that  popular  idea  (wonder- 
fully wide-spread  it  must  be  admitted)  that  at  the 
epochs  of  what  are  called,  most  illogically,  the 
Moon's  "  changes,"  changes  of  weather  may  cer- 
tainly be  expected.  There  is  absolutely  no 
foundation  whatever  for  this,  and  still  more  void 


100         THE   STORY  OF   THE   SOLAR .  SYSTEM. 

of  authority  (if  such  a  phrase  is  admissible)  is  a 
table  of  imaginary  weather  to  be  expected  at 
changes  of  the  Moon,  often  met  with  in  books 
published  half  a  century  ago,  and  still  occasion- 
ally reprinted  in  third-rate  almanacs,  and  desig- 
nated "Dr.  Herschel's  Weather  Table."  This 
precious  production  is  not  only  devoid  of  authen- 
ticity as  regards  its  name,  but  may  easily  be  seen 
to  be  fraudulent  in  its  reputed  facts  any  month  in 
the  year. 

It  would  be  beyond  both  my  present  available 
space  and  the  legitimate  objects  of  this  work  to 
attempt  even  an  outline  of  the  influences  over 
things  terrestrial  ascribed  to,  or  associated,  rightly 
or  wrongly,  with  the  Moon,  and  of  which  the 
word  "  lunatic  "  perhaps  affords  the  most  familiar 
exponent. 


CHAPTER   VII. 

MARS. 

MARS,  though  considerably  smaller  than  the 
Earth,  is  commonly  regarded  as  the  planet  which, 
taken  all  in  all,  bears  most  resemblance  to  the 
Earth,  though  only  one-fourth  its  size.  Under 
circumstances  which  have  already  been  briefly 
alluded  to  in  Chapter  L,  Mars  exhibits  from  time 
to  time  a  slight  phase,  but  nothing  approaching 
in  amount  the  phases  presented  by  the  two  in- 
ferior planets,  Mercury  and  Venus.  When  in  op- 
position to  the  Sun,  that  is  to  say  when  on  the 
meridian  at  midnight,  it  has  a  truly  circular  disc; 
but  between  opposition  and  its  two  positions  of 


MARS.  10 1 

quadrature  it  is  gibbous.  At  the  minimum  phase, 
which  is  at  each  quadrature,  E.  or  W.  as  the  case 
may  be,  the  planet  resembles  the  Moon  3  days 
from  its  "full."  These  phases  are  an  indication 
that  Mars  shines  by  the  reflected  light  of  the  Sun. 


FIG.  12.— Four  views  of  Mars  differing  90°  in  longitude  (Barnard). 


It  is  a  remarkable  tribute  to  Galileo's  powers  of 
observation  that  with  his  trumpery  telescope, 
only  a  few  inches  long,  he  should  have  been  able 
to  suspect  the  existence  of  a  Martial  phase. 
Writing  to  a  friend  in  1610  he  says: — "  I  dare  not 
affirm  that  I  can  observe  the  phases  of  Mars ; 
however,  if  I  mistake  not,  I  think  I  already  per- 
ceive that  he  is  not  perfectly  round." 

The  period  in  which  Mars  performs  its  journey 
round  the  Sun  (called  the  sidereal  period)  is  about 
687  days  ;  but  owing  to  the  Earth's  motion  we 
are  more  concerned  with  what  is  called  the 


102         THE  STORY  OF  THE   SOLAR  SYSTEM. 

planet's  synodical  period  of  780  days  than  with 
its  sidereal  period  of  687  days.  The  synodical 
period  is  the  interval  between  two  successive  con- 
junctions or  oppositions  of  the  planet  as  regards  - 
the  Earth,  and  780  days  being  twice  365  and  50 
days  over,  it  follows  that  we  have  an  opportunity 
of  seeing  the  planet  at  its  best  about  every  2 
years;  and  this  is  one  of  the  reasons  why  Mars 
has  been  so  much  and  so  thoroughly  studied  as 
regards  its  physical  appearance.  Of  course  Mars 
is  not  equally  well  seen  every  2  years,  because  it 
may  so  happen  at  a  given  opposition  that  it  may 
be  at  its  nearest  to  the  Sun  (perihelion),  and  the 
Earth  at  its  farthest  from  the  Sun  (aphelion),  in 
which  case  the  actual  distance  between  the  two 
bodies  will  be  the  greatest  possible.  What  is 
therefore  wanted  is  for  the  planet  to  be  nearest 
to  the  Sun  and  nearest  to  the  Earth  at  the  same 
time,  under  which  circumstances  it  shines  with  a 
brilliancy  rivalling  Jupiter.  This  favourable  com- 
bination occurs  once  in  7  synodical  revolutions, 
or  about  every  15  years.  The  most  favourable 
oppositions  occur  at  the  end  of  August,  and  the 
least  favourable  at  the  end  of  February.  The  next 
very  favourable  opposition  will  not  occur  until 
1909.  Mars  may  approach  to  within  about  35 
millions  of  miles  from  the  Earth  at  a  favourable 
opposition,  whilst  under  extreme  circumstances 
the  other  way  it  may  be  no  nearer  than  61  millions 
of  miles  at  opposition. 

Mars  in  opposition  is  a  very  conspicuous  ob- 
ject in  the  Heavens,  shining  with  a  fiery  red  light 
which  has  always  been  regarded  as  a  peculiar 
attribute  of  the  planet,  so  much  so  that  its  name, 
or  epithet,  in  many  languages  conveys  the  idea 
of  "  fiery  "  or  "  blazing."  It  is  recorded  that  in 


MARS.  103 

August  1719  its  brilliancy  was  such  as  to  cause 
a  panic  amongst  the  public. 

Telescopically  examined,  Mars  is  always  found 
to  exhibit  patches  of  shade  of  various  sizes  and 
shapes,  and,  on  the  whole,  fairly  permanent  from 
year  to  year.  During  the  last  few  years  in  par- 
ticular these  markings  have  been  subjected  to 
very  careful  scrutiny  and  measurement  at  the 
hands  of  numerous  observers  ot  skill  and  experi- 
ence, and  armed  in  many  cases  with  very  power- 
ful telescopes.  The  conjoint  effect  of  the  ob- 
servations obtained  has  been  largely  to  augment 
our  knowledge  of  the  planet's  geography,  or  (to 
use  the  proper  term)  k<  areography."  Before  de- 
scribing the  minutest  details  recorded  and  pen- 
cilled by  the  best  observers,  it  will  be  best  to 
speak  of  the  leading  general  features  which  are 
within  the  grasp  of  comparatively  small  telescopes 
— say,  refractors  of  6  inches  and  reflectors  of  12 
inches  in  aperture.  The  first  thing  which  pre- 
sents itself  as  very  obvious  on  the  disc  of  Mars, 
is  the  fact  that  certain  portions  are  ruddy,  whilst 
others  are  greenish  in  hue.  It  is  generally  as- 
sumed that  the  red  areas  represent  land  and  the 
green  areas  water.  On  this  subject  Sir  John 
Herschel's  remarks,  penned  about  half  a  century 
ago,  may  be  said  still  to  stand  good  He  ascribes 
the  ruddy  colour  to  "  an  ochrey  tinge  in  the  gen- 
eral soil,  like  what  the  red  sandstone  districts 
on  the  Earth  may  possibly  offer  to  the  inhab- 
itants of  Mars,  only  more  decided."  The  pro- 
priety of  this  thought  will  be  best  appreciated 
by  a  reader  who  has  travelled  through  parts  of 
North  Gloucestershire,  and  seen  a  succession  of 
ploughed  fields  in  that  locality.  The  deep  red 
colour  of  the  soil  is  in  many  places  very  con- 


104         THE   STORY   OF  THE  SOLAR  SYSTEM. 

spicuous.  It  has  often  been  remarked  that  the 
redness  of  Mars  is  much  more  noticeable  with  the 
naked  eye  than  with  a  telescope ;  and  Arago  car- 
ried this  idea  one  step  further  in  suggesting  that 
the  higher  the  optical  power  the  less  the  colour. 
This,  however,  might  naturally  be  expected. 

The  most  prominent  surface  marking  on  Mars 
is  that  known  as  the  "  Kaiser  Sea,"  sometimes 
called  the  "V-mark"  from  its  resemblance  to 
that  letter,  though  a  leg  of  mutton  would  be 
quite  as  good  a  simile.  East  of  the  Kaiser  Sea 
and  a  little  north  of  the  planet's  equator  is  a 
well-defined  dark  streak  known  as  "  Herschel 
II.  Strait";  whilst  on  the  west  side  is  another 
shaded  area  which  has  been  called  "  Flammarion 
Sea."  These  three  features  are  so  very  conspic- 
uous, that,  provided  the  hemisphere  in  which  they 
are  situated  is  fairly  in  front  of  the  observer, 
his  telescope,  if  it  will  show  anything  on  Mars, 
will  show  these.  The  white  patches  seen  on 
certain  occasions  at  Mars's  N.  pole  and  close  to  its 
S.  pole  form  another  special  feature  of  interest 
connected  with  this  planet.  It  admits  of  no 
doubt  whatever  that  these  are  immense  masses 
of  snow  and  ice  which  undergo  at  stated  intervals 
changes  analogous  to  the  changes  which  we  know 
happen  in  the  great  fields  of  ice  situated  in  the 
regions  of  the  Earth  surrounding  the  Earth's  two 
poles.  Not  only  do  these  white  patches  look 
like  snow,  but  if  attention  is  paid  to  the  changes 
they  undergo  and  the  epochs  at  which  the  changes 
take  place  there  will  be  found  abundant  confir- 
mation of  this  theory,  for  these  patches  decrease 
in  size  when  brought  under  the  Sun's  influence 
on  the  approach  of  summer  and  increase  again  in 
size  when  the  summer  is  over  and  winter  draws 


MARS.  105 

near.  In  the  second  half  of  1892  the  Southern 
Pole  was  in  full  view,  and  during  especially  July 
and  August  the  diminution  of  the  snow  area  from 
week  to  week  was  very  evident.  Schiaparelli, 
who  observed  it  with  great  attention  during  that 
season,  noted  at  the  commencement  of  the  season 
that  the  snow  reached  at  the  first  as  far  as  lati- 
tude 70°  and  formed  a  polar  cap  some  1200  miles 
in  diameter.  Its  subsequent  decrease,  however, 
was  so  marked  that  two  or  three  months  later  the 
diameter  of  the  snow  patch  had  dwindled  to  no 
more  than  180  miles,  and  became  indeed  still 
smaller  at  a  later  period.  The  summer  solstice 
on  Mars  occurred  on  October  13,  1892,  which  was 
therefore  the  epoch  of  midsummer  for  Mars's 
southern  hemisphere.  Whilst  these  changes  were 
taking  place  in  the  southern  hemisphere,  no  doubt 
changes  of  the  reverse  character  were  going  on  in 
the  northern  hemisphere,  but  they  were  not  visi- 
ble from  the  Earth  because  the  North  Pole  was 
situated  in  that  hemisphere  of  Mars  which  was 
turned  away  from  the  Earth.  In  previous  years, 
however,  the  North  Pole  being  turned  towards  the 
Earth  its  snow  was  also  seen  to  undergo  the  same 
sort  of  change  ;  in  other  words,  was  seen  to  melt. 
This  happened,  and  was  seen  in  1882,  1884,  and 
1886.  These  observations  of  the  alternate  in- 
crease and  decrease  of  the  polar  snow  on  Mars 
may  be  viewed  with  telescopes  of  moderate  power, 
but  of  course  it  is  more  interesting  and  profitable 
to  watch  them  with  a  large  telescope.  The  fact 
(for  it  is  an  undoubted  fact)  that  the  north  polar 
snow  is  concentric  with  the  planet's  axis  whilst 
the  southern  polar  patch  is  eccentric  to  the  extent 
of  about  180  miles  from  the  southern  pole  is  one 
which  has  not  yet  received  a  satisfactory  explana- 


106          THE   STORY   OF  THE   SOLAR   SYSTEM. 

tion.  If  both  patches  were  eccentric  so  as  to  be 
exactly  opposite  to  one  another  an  explanation 
wpuld  be  much  more  easy  for  we  might  say  that 
the  poles  of  rotation  lay  in  one  direction  and  the 
poles  of  cold  in  another. 

I  have  spoken  on  a  previous  page  of  three 
specially  conspicuous  shadings  of  Mars,  and  other 
similar  shadings  to  the  number  perhaps  of  a 
couple  of  dozen  were  generally  recognised  by 
astronomers  (having  been  mapped  and  named) 
down  to  about  the  year  1877.  In  that  year  the 
astronomical  world  was  startled  by  the  announce- 
ment that  Schiaparelli  of  Milan,  an  able  and 
competent  observer,  had  discovered  that  those 
shaded  areas  which  all  previous  astronomers  had 
regarded  as  continents  or  vast  tracts  of  land, 
were  in  reality  islands,  that  is  to  say,  so  far,  that 
the  continents  in  question  were  cut  up  by  in- 
numerable channels  intersecting  one  another  at 
various  angles.  When  this  discovery  was  an- 
nounced, and  older  observations  and  drawings 
came  to  be  examined,  it  was  found,  or  at  any- 
rate  thought,  that  these  so-called  canals  might  be 
traced  in  drawings  of  earlier  dates  by  Dawes, 
Secchi,  and  Holden.  So  much  for  1877.  In  De- 
cember, 1881,  the  planet  was  again  in  opposition, 
but  farther  off  in  distance,  and  therefore  smaller 
in  size  than  in  1877.  It  was,  however,  higher  up 
in  the  Heavens  as  seen  at  Milan  and  the  weather 
appears  to  have  been  more  favourable..  In  these 
altered  circumstances  Schiaparelli  again  saw  his 
canals,  but  this  time  they  were  in  at  least  as  many 
as  twenty  instances  seen  in  duplicate  ;  that  is  to 
say,  a  twin  canal  was  seen  to  run  parallel  to  the 
original  one  at  a  distance  of  from  200  to  400 
miles,  as  the  case  mig^t  be.  The  existence  of 


MARS.  107 

not  only  single  canals  but  of  twin  canals  seems 
an  established  fact,  for  Schiaparelli's  drawings 
and  descriptions  have  been  confirmed  by  compe- 


FIG.  13. — Mars,  August  27,  1892  (Guiot). 

tent  testimony ;  but  explanation  is  nowhere;  es- 
pecially in  view  of  Schiaparelli's  own  idea  that 
the  duplication  of  his  canals  is  perhaps  not  a  per- 
manent feature  but  a  periodical  phenomenon  de- 
pending on,  or  connected  in  some  way  with,  Mars's 
seasons. 

Several  points  stand  out  clearly  established 
by  the  observations  of  Mars  during  the  opposition 
of  1894,  especially  the  correctness  of  Schiaparelli's 
discoveries  and  maps.  Most  of  the  canals  origi- 
nally seen  by  him  were  again  seen,  and  thus  their 
existence  was  confirmed,  whilst  new  ones  were 
also  noticed.  Many  of  these  canals  were  double. 


Io8         THE   STORY   OF   THE   SOLAR   SYSTEM. 

The  great  extent  of  the  S.  Polar  cap  and  its  rapid 
disappearance  as  Mars's  summer  approached  was 
also  a  special  feature  of  the  observations  of  1894. 
It  dwindled  until  it  became  almost  invisible,  or  at 
best  showed  as  a  tiny  speck.  It  is  thought  by 
some  observers  that  as  the  Polar  cap  melts,  the 
water  collects  round  the  Pole,  and  thence  flows 
over  the  planet's  surface,  giving  rise  to  the  phe- 
nomenon of  canals,  and  that  this  is  the  way  the 
planet's  surface  is  irrigated.  It  may  here  be  re- 
marked that  the  word  "  canal,"  which  has  been 
given  to  these  dark  streaks  crossing  and  cutting 
up  the  large  areas  of  land  in  Mars,  is  an  unfor- 
tunate one,  suggesting  as  it  does  artificial  agency. 
But  these  Martial  canals  are  probably,  especially 
the  largest,  a  great  many  miles  in  wridth  and 
hundreds  of  miles  in  length,  though  some  are 
smaller;  and  they  are  probably  nature's  method 
of  distributing  over  the  continents  and  lands  of 
Mars  the  water  which  collects  round  the  Pole 
during  the  rapid  melting  of  the  Polar  snows. 

The  idea  of  the  presence  of  cloud  or  mist  on 
Mars  also  received  strong  confirmation  in  1894. 
Large  portions  of  the  planet's  disc  were  found  to 
be  hidden  from  view.  "Herschel  I.  Continent" 
and  the  "  Maraldi  Sea  "  (both  well-known  mark- 
ings on  Mars,  readily  visible  with  small  telescopes) 
were  at  times  quite  obscured  by  cloud.  Indeed, 
the  Maraldi  Sea  was  occasionally  quite  blotted 
out :  other  well-known  markings  were  also  either 
blotted  out  or  only  faintly  seen.  These  facts 
seem  almost  to  prove  conclusively  the  existence 
of  cloud  and  vapour  in  Mars,  especially  as  some 
of  these  markings  subsequently  again  assumed 
their  ordinary  form  and  colour.  Bright  projec- 
tions too  were  seen  at  times  on  the  terminator  of 


MARS.  109 

Mars,  giving  rise  to  the  belief  that  there  are  high 
mountains  on  the  planet,  though  some  observers 
regarded  these  projections  as  high  clouds  power- 
fully reflecting  the  Sun's  light. 

Mars  rotates  on  its  axis  in  24.^1.  37m.  225.,  a 
period  so  nearly  coincident  with  the  period  of  the 
Earth's  rotation  as  greatly  to  facilitate  the  map- 
ping of  Mars's  features  by  work  continued  from 
day  to  day  by  observers  who  have  the  necessary 
instrumental  means  and  artistic  skill  in  handling 
the  pencil. 

Mars  has  an  atmosphere  which  may  be  said  to 
be  no  more  than  moderately  dense  ;  that  is  to  say 
much  less  dense  than  the  Earth's  atmosphere.  Of 
course  the  existence  of  snow,  which  has  been 
taken  for  granted  on  a  previous  page,  carries  with 
it  the  existence  of  water  and  aqueous  vapour — a 
fact  capable  of  independent  spectroscopic  proof. 

The  inclination  of  Mars's  axis  to  the  ecliptic 
has  not  been  ascertained  with  all  desirable  cer- 
tainty, but  if  Sir  W.  Herschel's  estimate  that  the 
obliquity  on  Mars  is  28f°  (the  Earth's  obliquity 
being  23-^-°)  is  correct,  it  is  evident  that  there  must 
be  a  very  close  similarity  between  the  seasons  of 
the  Earth  and  the  seasons  of  Mars,  thereby  furnish- 
ing another  link  of  proof  to  support  the  statement 
made  at  the  commencement  of  this  chapter  that, 
taken  all  in  all,  Mars  is  the  planet  which  bears 
most  resemblance  to  the  Earth. 

The  apparent  absence  of  satellites  in  the  case 
of  Mars  was  long  a  matter  of  regret  to  astrono- 
mers ;  they  seemed  to  think  that  such  a  planet 
ought  to  have  at  least  one  companion.  At  last, 
in  1887,  two  were  found  by  Hall  at  Washington, 
U.  S.,  using  a  very  fine  refractor  of  26  inches 
aperture.  These  satellites,  which  have  been 


110         THE  STORY  OF  THE  SOLAR  SYSTEM. 

named  Phobos  and  Deimos,  are,  however,  very 
small,  for  Phobos  at  its  best  only  resembles  a  star 
of  mag.  ii-j-,  whilst  Deimos  is  no  brighter  than 
a  star  of  mag.  13**;  from  this  it  will  be  under- 
stood that  only  very  large  telescopes  will  show 
either  of  them.  Phobos  revolves  round  Mars  in 
7^-  hours  at  a  distance  of  about  6000  miles,  w'hilst 
Deimos  revolves  in  30  hours  at  a  distance  of 
about  15,000  miles.  It  has  been  thought  that 
neither  of  them  can  be  more  than  about  6  or  7 
miles  in  diameter,  and  therefore  that  they  can 
not  afford  much  light  to  their  primary. 

Mars  revolves  round  the  Sun  in  686d.  230. 
3om.,  at  a  mean  distance  of  141  million  of  miles, 
which  the  eccentricity  of  its  orbit  may  increase 
to  154  millions  or  diminish  to  128  millions.  The 
planet's  apparent  diameter  varies  between  4"  in 
conjunction  and  30''  in  opposition.  Owing  to  the 
great  eccentricity  of  the  orbit  the  planet's  appar- 
ent diameter  as  seen  from  the  Earth  varies  very 
much  at  different  oppositions.  The  real  diameter 
is  rather  more  than  4000  miles. 


CHAPTER   VIII. 

THE    MINOR    PLANETS. 

IN  1772  a  German  astronomer  named  Bode, 
of  Berlin,  drew  attention  to  certain  curious  nu- 
merical relations  subsisting  between  the  distances 
of  the  various  planets.  This  "  law,"  as  it  has 
been  sometimes  called,  usually  bears  Bode's  name, 
though  it  was  not  he  but  J.  D.  Titius  of  Wittem- 
berg  who  really  first  discovered  it. 


THE   MINOR   PLANETS. 


Ill 


Take  the  numbers — 

o,  3,  6,  12,  24,  48,  06,  192,  384; 
each  of  which  (the  second  excepted)  is  double  the 
preceding ;  adding  to  each  of  these  numbers  4  we 
obtain — 

4,  7,  10,  16,  28,  52,  100,  196,  388; 
which  numbers  approximately  represent  the  dis- 
tances of  the  planets  from  the  sun  expressed  in 
radii   of   the    Earth's    orbit.      A   little   table   will 
make  the  matter  more  clear. 


Planets. 

Distance  : 
Bode's  Law. 

True  distance 
from  Sun. 

Mercury 

4 

3-9 

Venus 

7 

7-2 

Earth 

10 

10.  0 

Mars 

16 

15.2 

[Ceres] 

[28] 

[27.7] 

Jupiter 
Saturn 

52 

100 

52.0 
95-4 

[Uranus] 
[Neptune] 

[196] 
[388] 

[191.8] 
[300.0] 

Bode  having  examined  these  relations  and 
noticing  the  void  between  16  and  52  (Ceres  and 
the  other  minor  planets,  and  Uranus  and  Neptune 
also,  being  then  unknown)  ventured  to  predict 
the  discovery  of  new  planets,  and  this  idea  stimu- 
lated him  to  organise  a  little  company  of  astron- 
omers to  hunt  for  new  planets.  Before,  however, 
this  scheme  was  got  into  working  order,  Piazzi, 
director  of  the  Observatory  at  Palermo,  on  Jan- 
uary i,  1801,  noted  an  8th  magnitude  star  in 
Taurus,  which  on  the  next  and  succeeding  nights 
he  saw  again,  and  found  had  moved.  He  ob- 
served the  strange  object  for  6  weeks,  when  ill- 
8 


112          THE  STORY  OF  THE   SOLAR  SYSTEM. 

ness  interrupted  him.  However  he  wrote  letters 
announcing  what  he  had  seen,  one  of  them  to 
Bode  himself;  but  this  letter,  though  dated  Jan. 
24,  did  not  reach  Bode  at  Berlin,  till  March  20 — 
a  striking  illustration  of  the  state  of  the  Postal 
service  on  the  Continent  less  than  100  years  ago. 
The  new  body,  at  first  assumed  to  be  a  tailless 
comet,  was  eventually  recognised  to  be  a  new 
planet;  and  the  name  of  Ceres,  the  tutelary 
goddess  of  Sicily,  was  at  Piazzi's  instance  be- 
stowed upon  it. 

Looking  for  Ceres  in  March,  1802,  Olbers  at 
Bremen,  came  upon  another  new  planet,  which 
was  afterwards  named  Pallas.  At  first  he  thought 
he  had  got  hold  of  a  new  variable  star,  but  two 
hours  sufficed  to  show  that  the  object  under 
notice  was  in  motion.  The  two  new  bodies  were 
found  to  be  so  much  alike  in  size  and  appearance, 
and  in  their  orbits,  that  Olbers  suggested  both 
were  but  fragments  of  some  larger  body  wrhich 
had  been  shattered  by  some  great  convulsion  of 
nature.  The  idea  was  a  daring  one,  and  it  was 
an  attractive  one,  though  now  regarded  as  un- 
tenable. However  it  served  the  purpose  of  stim- 
ulating research,  and  the  discovery  of  Pallas  was 
followed  by  that  of  Juno,  by  Harding,  at  Lilien- 
thal  1804;  and  of  Vesta,  by  Olbers,  at  Bremen  in 
1807. 

The  organised  search  for  minor  planets  was 
relinquished  in  1816,  presumably  because  no 
more  planets  seemed  to  be  forthcoming,  and  it 
does  not  appear  that  any  further  attempts  were 
made  by  anybody  till  about  1830,  when  a  Prussian 
amateur,  named  Hencke  of  Driessen,  profiting  by 
the  publication  of  some  new  star  maps  put  forth 
by  the  Berlin  Academy,  commenced  a  methodical 


THE   MINOR   PLANETS.  113 

search  for  small  planets.  These  Berlin  maps,  one 
for  each  hour  of  R.  A.,  were  only  completed  in 
1859,  and,  therefore,  Hencke  had  only  a  small 
number  of  them  at  his  command  during  the  early 
years  of  his  labours.  Still  it  is  strange  that  15 
years  elapsed  before  his  zeal  and  perseverance 
were  rewarded,  his  first  discovery,  the  planet 
Astraea,  not  taking  place  till  December  1845. 
Once  however  the  ice  was  broken  new  planets 
followed  with  considerable  rapidity,  and  begin- 
ning with  1847,  no  year  has  elapsed  without  sev- 
eral or  many  having  been  found.  During  the 
last  decade  the  number  detected  annually  has 
been  very  great — sometimes  as  many  as  20  in  a 
year,  but  this  has  been  the  result  of  photography 
being  brought  to  bear  on  the  work.  It  is  obvious 
that  if  a  photograph  of  a  given  field  taken  on 
any  one  day  is  compared  with  a  photograph 
taken  a  few  days  earlier  or  later,  and  any  of  the 
objects  photographed  have  moved,  their  change 
of  place  will  soon  be  noticed  and  will  be  a  dis- 
tinct proof  of  their  planetary  nature. 

It  seems  quite  certain  that  all  the  larger  of 
these  planets  have  now  been  found,  for  the  aver- 
age brilliancy  (and  this  no  doubt  means  the 
average  size)  of  those  recently  discovered  has 
been  steadily  diminishing  year  by  year,  and  it 
looks  as  if  the  limit  of  visibility  will  soon  be 
reached,  if  it  has  not  been  reached  already. 

The  three  largest  of  these  bodies,  in  order  of 
size,  have  generally  been  thought  to  be  Vesta, 
Ceres,  and  Pallas;  but  Barnard,  from  observa- 
tions made  in  1894,  concluded  that  Ceres  is  520 
miles  in  diameter ;  Pallas,  304  miles ;  and  Vesta, 
241  miles.  As  to  all  the  rest  of  the  minor  planets, 
excepting  Juno,  Hornstein  is  of  opinion  that 


(t 

114        THE  STORY  OF  THE  SOLAR  SYSTEM. 

those  having  a  greater  diameter  than  25  geo- 
graphical miles  are  few  in  number,  and  that  the 
majority  of  them  are  no  larger  than  from  5  to  15 
miles  in  diameter. 

From  what  has  gone  before  the  reader  will 
readily  infer  that  these  minor  planets  are  of  no 
sort  of  interest  to  the  casual  amateur  who  dabbles 
in  Astronomy  ;  and  indeed  that  they  are  of  very 
little  interest  to  anybody.  With  a  few  general 
statistics,  therefore,  this  chapter  may  be  con- 
cluded. The  total  number  of  minor  planets  now 
known  nearly  reaches  500,  and  every  year  in- 
creases the  list;  but  not,  however,  at  as  rapid  a 
rate  as  was  once  the  case,  because  the  German 
mathematicians,  who  alone  latterly  have  been 
willing  to  trouble  themselves  with  the  computa- 
tion of  the  orbits,  are  understood  to  have  an- 
nounced that  they  are  no  longer  able  to  keep 
pace  with  the  discoveries  made.  Those  who  care 
to  investigate  in  detail  the  circumstances  of  these 
planets  will  find  great  extremes  in  the  nature  of 
the  orbits.  Whilst  the  planet  nearest  to  the  Sun 
has  a  period  of  only  3  years,  the  most  distant  oc- 
cupies nearly  9  years  in  performing  its  journey 
round  the  Sun.  So,  also,  there  are  great  differ- 
ences in  the  eccentricities  of  the  orbits  and  in 
their  inclinations  to  the  ecliptic.  Whilst  one 
planet  revolves  almost  in  the  plane  of  the  ecliptic, 
another  (Pallas)  has  an  orbit  which  is  inclined  no 
less  than  34°  to  the  ecliptic.  One  word,  in  con- 
clusion, as  to  the  names  applied  to  these  bodies. 
At  the  outset  the  names  given  were,  without  ex- 
ception, chosen  from  the  mythologies  of  ancient 
Greece  and  Rome,  but,  latterly,  the  most  fantastic 
and  ridiculous  names  have  in  many  cases  been 
selected,  names  which  in  too  many  instances  have 


JUPITER.  115 

served  no  other  purpose  than  that  of  displaying 
the  national  or  personal  vanity  of  the  astronomers 
who  applied  them  to  the  several  planets.  The 
French  are  great  offenders  in  this  matter. 


CHAPTER   IX. 

JUPITER. 

THE  planet  Jupiter  occupies,  in  one  sense,  the 
first  position  in  the  planetary  world,  it  being  the 
largest  of  all  the  planets  Moreover,  with  the  ex- 
ception of  Venus,  it  is  the  brightest  of  the  planets. 
As  with  Mars,  and  for  the  like  reason,  Jupiter, 
when  in  the  positions  known  as  the  Quadratures 
(or  near  thereto),  exhibits  a  slight  phase,  but 
owing  to  the  far  greater  distance  from  the  Sun  of 
Jupiter,  compared  with  Mars,  the  deviation  of  the 
illuminated  surface  from  that  of  a  complete  circle 
is  very  small ;  it  is,  however,  perceptible  at  or 
near  the  time  of  quadrature,  a  slight  shading  off 
of  the  limb  farthest  from  the  Sun  being  trace- 
able. 

Jupiter  is  noteworthy  on  account  of  two  fea- 
tures, both  of  them  more  or  less  familiar,  at  least 
by  name,  to  most  people — its  belts  and  its  satel- 
lites,— both  of  which  will  be  described  in  due 
course. 

The  belts  are  dusky  streaks,  which  vary  from 
time  to  time  both  in  breadth  and  number:  most 
commonly  two  broad  belts  will  be  seen  with  two 
or  three  narrower  ones  on  either  side;  but  some- 
times all  are  rather  narrow,  and  their  narrowness 
is  made  up  for  by  an  increa.se  in  their  number. 


Il6          THE  STORY  OF  THE   SOLAR   SYSTEM. 

Under  all  circumstances  they  lie  practically  par- 
allel, or  nearly  so,  to  the  planet's  equator.  It  is 
generally  thought  that  the  planet,  whatever  may 
be  its  actual  structure  or  constitution,  is  sur- 
rounded by  a  dense  cloudy  envelope,  and  that  the 


FIG.  14.— Jupiter,  November  27,  1857  (Dawes). 

shaded  streaks  which  we  call  belts  are  rifts  in  this 
atmosphere,  which  expose  to  view  the  solid  body 
of  the  planet  underneath.  Whether,  however,  the 
term  "  solid  body  "  is  an  accurate  one  to  be  used 
in  this  connection  is  thought  by  some  to  be  open 
to  doubt.  The  laws  which  regulate  the  existence 
of  these  belts  are  quite  unknown  ;  indeed  it  seems 
doubtful  whether  any  laws  exist  at  all,  for  the 
belts  at  one  time  .appear  to  undergo  constant 
change,  whilst  at  another  time  they  remain  almost 
unchanged  for  several  months.  It  has  been  sug- 
gested that  when  the  changes  are  rapid  it  must 
t>e  presumed  that  great  atmospheric  storms  a.re. 


JUPITER.  117 

to  be  considered  as  in  progress,  and  possibly  this 
may  be  the  true  explanation.  Belts  are  commonly 
non-existent  immediately  under  the  equator ; 
whilst  north  and  south  of  this  void  space  it  most 
usually  happens  that  there  is  one  broad  belt  and 
several  narrower  ones  in  each  hemisphere.  At 
each  pole  the  planet's  brightness  is  less  than  the 
average  brightness,  but  it  cannot  exactly  be  said 
that  this  is  due  to  the  existence  there  of  belts 
properly  so  called. 

It  was  formerly  considered  that  no  tinges  of 
colour  could  be  traced  on  Jupiter  except  a  silvery 
gray  of  different  degrees  of  intensity  ;  but  during 
the  last  thirty  years  there  can  be  no  doubt  that 
shades  of  brown,  red,  and  orange,  of  no  great 
depth,  but  yet  quite  definite  have  been  traceable. 
Many  observers  concur  in  this  opinion.  Whether 
this  detection  of  colour  is  due  to  an  absolute  de- 
velopment of  colour  during  the  period  in  ques- 
tion ;  or  whether  its  detection  is  merely  the  result 
of  more  careful  scrutiny  with  better  instruments 
is  a  matter  as  to  which  the  evidence  is  not  clear. 
Though  the  general  position  of  the  belts  is  such 
that  they  are  parallel  to  the  planet's  equator,  yet 
there  are  sometimes  exceptions  to  this  rule,  for 
in  a  few  very  rare  instances  a  streak  in  the 
nature  of  a  narrow  belt  has  been  seen,  inclined  to 
the  equator  at  a  decided  angle,  perhaps  20°  or 
even  more. 

It  occasionally  happens  that  spots  are  seen  on 
Jupiter's  belts.  Sometimes  these  remain  visible 
for  a  considerable  period.  They  are  either  dark 
or  luminous,  and  their  origin  is  unknown.  Besides 
these  casual  spots,  which  are  always  small  in  size, 
there  was  visible  during  many  years  following 
1878  a  very  remarkable  and  conspicuous  large 


Il8         THE   STORY  OF  THE   SOLAR  SYSTEM. 

spot,  strongly  red  in  colour  for  several  years, 
though  it  afterwards  became  much  fainter.  This 
spot  exhibited  an  oval  outline  and  was  about 
27,000  miles  long  and  8000  miles  broad.  For 
about  4  years  it  maintained  its  intense  red  colour 
and  its  shape  almost  unaltered;  but  after  1882, 
the  shape  remaining,  the  colour  sensibly  faded. 
The  observations  which  were  made  on  this  spot 
during  1886  by  Professor  Hough  at  Chicago, 
U.  S.,  with  an  i8-inch  refractor,  led  him  to  the 
opinion  that  the  persistence  of  the  red  spot  for 
so  many  years  rendered  untenable  the  generally 
accepted  theory  that  the  phenomena  seen  on  the 
surface  of  the  planet  are  due  to  atmospheric 
causes. 

Some  astronomers  have  thought  that  a  rela- 
tionship subsists  between  the  spots  on  the  Sun 
and  the  spots  on  Jupiter.  There  certainly  seems 
an  apparent  identity  in  point  of  time  between  the 
two  classes  of  spots,  and  on  the  assumption  that 
the  spots  on  Jupiter  are  indicative  of  disturb- 
ances on  the  planet,  Ranyard  broached  the  idea 
that  both  classes  of  phenomena  are  dependent  on 
some  extraneous  cosmical  change ;  and  are  not 
related  as  cause  and  effect.  Browning  suggested 
many  years  ago  that  the  red  colour  of  the  belts 
is  a  periodical  phenomenon  coinciding  with  the 
epoch  of  the  greatest  display  of  sunspots,  but  this 
thought  does  not  appear  to  have  been  followed  up 
by  any  one.  Spots  on  Jupiter  seem  to  have  been 
first  recorded  by  Robert  Hooke  in  1664.  In  the 
following  year  Cassini  saw  a  spot  which  he  found 
to  be  in  motion,  and  by  following  it  attentively 
he  inferred  that  the  planet  rotated  on  its  axis  in 
9h.  56m.  It  is  a  remarkable  illustration  of  the 
great  care  bestowed  by  Cassini  on  his  astronom- 


JUPITER.  119 

ical  work  that  the  best  modern  determinations  of 
Jupiter's  rotation-period  differ  from  Cassini's  esti- 
mate by  only  half  a  minute. 

Bearing  in  mind  the  enormous  size  of  Jupiter 
compared  with  the  Earth,  whilst"  its  period  of  ro- 
tation is  considerably  less  than  half  the  Earth's, 
it  will  be  at  once  seen  that  the  velocity  of  matter 
at  the  planet's  equator  is  immensely  great — 466 
miles  per  minute  against  the  Earth's  17  miles  per 
minute.  One  result  of  this  is  the  great  intensity 
of  the  centrifugal  force  at  the  equator,  and  like- 
wise the  greatness  of  the  compression  of  the 
planet's  body  at  the  poles.  Hind  has  suggested 
that  the  great  velocity  which  thus  evidently  exists 
may  have  the  effect,  by  reason  of  the  develop- 
ment of  the  heat  which  it  gives  rise  to,  of  com- 
pensating the  planet  for  the  small  amount  of 
heat  which  owing  to  its  distance  it  receives  from 
the  Sun. 

On  favourable  occasions  the  brilliancy  of  Ju- 
piter is  very  considerable ;  so  much  so  that  it 
rivals  Venus  and  Mars.  And  besides  this,  there 
appears  to  be  something  special  in  the  nature  of 
Jupiter's  surface,  for  not  only  does  it  seem  to 
radiate  a  much  larger  proportion  of  the  solar 
light  which  falls  on  it  than  do  the  planets  gener- 
ally, but  some  observers  have  expressed  the  opin- 
ion that  it  possesses  inherent  light  of  its  own. 
Speculations,  however,  such  as  this  must  always 
be  received  with  reserve,  because  of  the  evident 
difficulty  of  making  sure  of  the  facts  on  which 
they  must  be  based.  One  thing,  however,  seems 
less  open  to  doubt.  Bearing  in  mind  the  small 
amount  of  heat  which  reaches  Jupiter  from  the 
Sun,  there  is  reason  to  infer  that  the  clouds  which 
certainly  exist  on  Jupiter  must  owe  tneir  origin  to 


120         THE  STORY  OF  THE  SOLAR  SYSTEM. 

the  influence  of  some  other  heat  than  solar  heat; 
in  other  words  that  Jupiter  possesses  sources  of 
heat  within  itself. 

Jupiter  has  satellites,  5  in  number.  The  dis- 
covery of  four  of  these,  was  one  of  the  first  fruits 
of  the  invention  of  the  telescope,  for  they  were 
found  by  Galileo  in  January,  1610.  The  5th  sat- 
ellite is  so  small  that  it  escaped  notice  until  as 
recently  as  1892,  having  been  discovered  on  Sep-  * 
tember  9  of  that  year  by  Professor  Barnard,  with 
the  great  Lick  telescope  in  California.  It  is,  how- 
ever, so  minute  that  one  can  count  on  one's  fin- 
gers the  telescopes  capable  of  showing  it. 

The  four  old  satellites  of  Jupiter  shine  as 
stars  of  about  the  yth  magnitude;  in  other  words, 
they  are  sufficiently  bright  to  be  visible  with  tele- 
scopes however  small :  indeed  several  instances 
are  on  record  of  persons  gifted  with  very  good 
sight,  having  been  able  to  see  them  with  the  naked 
eye.  For  the  study  of  their  physical  appearance 
very  powerful  optical  assistance  is  necessary,  but 
their  movements  are  so  rapid,  and  the  phenomena 
which  result  from  those  movements  are  so  inter- 
esting, that  these  bodies  may  be  considered  to 
occupy  the  first  place  in  the  stock-in-trade  of 
every  amateur  astronomer,  who  lays  himself  out 
for  planet-gazing,  with  the  object  of  profiting 
himself  or  his  friends.  The  phenomena  here 
alluded  to  are  known  as  eclipses,  transits,  tand 
occupations. 

The  four  old  satellites  do  not  bear  any  names, 
but  are  numbered  from  the  innermost  outwards, 
and  are  always  alluded  to  by  their  numbers  as  I, 
II,  III  and  IV. 

An  eclipse  of  a  Jovian  satellite  is  identical  in 
principle  with  an  eclipse  of  the  Moon  ;  that  is  to 


JUPITER.  121 

say,  just  as  an  eclipse  of  the  Moon  happens  when 
the  Moon  passes  into  and  is  lost  in  the  Earth's 
shadow,  so  an  eclipse  of  a  Jovian  satellite  hap- 
pens when  such  satellite  becomes  lost  in  the 
shadow  cast  by  the  planet  into  space.  The  1st 
Ilnd  and  Illrd  satellites  in  consequence  of  the 
smallness  of  the  inclination  of  their  orbits,  un- 
dergo eclipse  once  in  every  revolution  round  their 
primary,  but  the  IVth  is  less  often  eclipsed,  owing 
to  the  joint  effect  of  its  considerable  orbital  incli- 
nation, and  of  the  distance  to  which  it  recedes  from 
its  primary. 

An  occultation  of  a  Jovian  satellite  is  akin  in 
principle  to  an  occultation  of  a  star  by  the  Moon. 
As  the  Moon  moving  forwards  suddenly  covers  a 
star,  so  the  planet,  on  occasions,  suddenly  covers 
one  of  its  satellites.  If  the  satellite  in  question 
is  the  IVth,  its  disappearance  behind  the  planet 
and  its  reappearance  from  behind  the  planet  will 
both  be  visible  in  due  succession.  This  is  often 
true  also  of  the  Illrd  satellite,  but  for  reasons 
connected  with  the  proximity  to  their  primary  of 
the  1st  and  Ilnd  satellites,  only  their  disappear- 
ance or  reappearance  (not  both)  can,  as  a  rule,  be 
observed  on  the  same  occasion.  The  most  inter- 
esting, by  far,  however,  of  the  phenomena  con- 
nected with  Jupiter's  satellites  are  their  transits 
in  front  of,  that  is  across,  the  visible  disc  of  the 
planet.  Though  these  transits  are  of  frequent 
occurrence,  yet  they  are  always  interesting  be- 
cause of  the  diverse  appearances  which  the  satel- 
lites exhibit  at  different  times,  and  which  cannot 
be  said  to  be  in  accordance  with  any  recognised 
laws.  Moreover,  in  observing  the  transit  of  a 
satellite,  we  may  often  see  the  black  shadow  cast 
by  the  satellite  on  the  planet's  disc ;  and  this 


122         THE  STORY  OF  THE   SOLAR   SYSTEM. 

shadow  will  sometimes  precede  and  sometimes 
follow  the  satellite  itself.  From  the  fact  that  the 
satellite  generally  appears  as  a  bright  spot  on  a 
bright  background  whilst  the  shadow  is  black,  or 
blackish,  an  inexperienced  observer  is  apt  to 
look  at  the  shadow  and  think  he  is  seeing  the 
satellite. 

Jupiter  revolves  round  the  Sun  in  not  quite  12 
years  at  a  mean  distance  of  483  millions  of  miles. 
Its  apparent  diameter  varies  between  50"  and  30" 
according  to  its  position  with  respect  to  the  Earth. 
Its  true  diameter  is  about  88,000  miles.  Owing 
to  its  large  size  and  rapid  rotation,  as  has  already 
been  mentioned,  Jupiter  is  very  much  flattened  at 
the  poles.  The  amount  of  this  (the  polar  "  com- 
pression ''  as  it  is  called)  is  about  T*g-. 


CHAPTER   X. 

SATURN. 

NEXT  beyond  Jupiter,  proceeding  outwards 
from  the  Sun,  we  reach  the  planet  Saturn,  which 
beyond  any  doubt  is  the  most  beautiful  and  most 
interesting  of  all  the  planets.  Nobody  who  has 
ever  had  a  fairly  good  chance  of  seeing  it  can 
have  the  least  doubt  that  this  is  the  case.  Briefly 
stated  the  three  main  features  which  constitute  its 
claims  are  : — (i)  Its  belts,  (2)  its  rings,  (3)  its  sat' 
ellites. 

The  belts  of  Saturn  resemble  generally  those 
of  Jupiter,  but  they  are  more  faint  and  less 
changeable.  Their  physical  cause,  however,  may 
be  assumed  to  t>e  the  same.  Taking  the  planet 


SATURN. 


123 


as  a  whole,  it  may  be  said  that  its  ordinary  colour 
is  yellowish  white,  the  belts  inclining  to  grayish 
white;  though  the  dark  belts  have  often  been 
thought  to  exhibit  a  greenish  hue.  Lassell  con- 
sidered that  the  south  pole  is  generally  darker 
than  the  north  pole  and  more  blue  in  tinge. 

There  is  one  important  particular  in  which 
the  belts  of  Saturn  differ  from  those  of  Jupiter. 
Jupiter's  belts  are  straight,  whereas  Saturn's  are 
sensibly  curved.  Supposing,  as  is  probable,'  that 
Saturn's  belts  are  parallel  to  the  planet's  equator, 
then  we  must  assume  that  the  plane  of  this  equa- 
tor makes  a  rather  considerable  angle  with  the 
ecliptic.  Spots  on  Saturn  are  very  rare.  Whether 
Saturn  has  an  atmosphere  seems  uncertain,  or 
perhaps  it  may  be  said  that  one  has  not  been 
proved  to  exist  but  may  exist.  The  question  of 
polar  snow  is  also  uncertain,  but  Sir  W.  Herschel 


FIG.   15.— Saturn,  Jan.  26,  1889  (Antoniadi). 


thought  he  could  trace  changes  of  hue  at  the  poles 
which  might  be  due  to  the  melting  of  snow. 

It  is  usual  to  speak  of  the  planet  itself  under 


124        THE  STORY  OF  THE  SOLAR  SYSTEM. 

the  name  of  the  "  Ball  "  when  it  is  not  a  question 
of  referring  to  the  whole  Saturnian  system  collect- 
ively. In  consequence  of  its  distance  from  the 
Sun,  Saturn  undergoes  no  equivalent  to  a  phase; 
or  to  be  more  exact,  no  phase  can  be  detected, 
though  theoretically  when  the  planet  is  in  quad- 
rature the  disc  must  undergo  an  infinitesimally 
small  loss  of  light. 

Though  the  point  has  now-a-days  no  scientific 
importance,  it  may  perhaps  be  desirable  just  to 
make  a  brief  allusion  to  Sir  W.  Herschel's- curious 
theory  that  Saturn  was  seen  by  him  to  be  com- 
pressed not  only  at  the  poles  but  at  the  equator, 
so  that  it  resembled  a  parallelogram  with  the  cor- 
ners rounded  off.  It  is  difficult  to  imagine  what 
could  have  given  rise  to  this  strange  idea,  though, 
of  course,  Herschel's  good  faith  in  advancing  it 
cannot  be  called  in  question.  I  refer  to  it  because 
it  will  be  found  mentioned  in  so  many  books  on 
astronomy,  often  under  the  name  of  the  "  square- 
shouldered  "  figure  of  Saturn.  As  a  theory  it  may 
be  regarded  as  quite  exploded  in  consequence  of 
accurate  measures  by  Bessel,  Main  and  others  hav- 
ing conclusively  shown  that  the  form  of  the  ball 
does  not  depart  from  that  of  a  regular  spheroid. 

In  referring  to  Saturn  generally,  we  speak  of 
its  ring  in  the  singular  number,  but,  in  point  of 
fact,  there  are  several  rings — three  in  particular. 
The  principal  bright  ring  is  really  double,  and 
within  the  innermost  bright  ring  there  is  a  dusky 
one,  perfect  as  a  ring,  but  not  luminous  as  the 
outer  rings  are.  By  way  of  distinguishing  one 
ring  from  another,  it  is  usual  to  adopt  Struve's 
nomenclature,  whereby  the  outermost  bright  ring 
is  called  A,  the  inner  bright  ring  B,  and  the  dusky 
ring  C. 


SATURN.  125 

A  good  engraving  will  convey  more  fully  and 
more  clearly  an  idea  of  what  the  Saturnian  sys- 
tem consists  of  than  the  fullest  verbal  description 
will  do.  (See  Frontispiece^] 

To  the -earliest  astronomers  who  possessed  tel- 
escopes, Saturn  proved  a  great  puzzle,  because  it 
seemed  to  undergo  changes  of  shape  which  were 
quite  inexplicable  on  any  principles  then  known. 
Galileo,  when  first  he  saw  it,  thought  it  presented 
an  oval  outline  which  might  be  due  to  a  central 
planet  having  a  smaller  planet  on  each  side  of  it, 
and  accordingly  he  announced  to  his  friend,  Kep- 
ler, that  the  most  distant  planet  was  tergeminum  or 
tri-form.  But  greater  magnifying  power  led  him 
to  arrive  at  the  conclusion  that  the  planet  was  not 
a  triple  combination  of  spheres,  but  one  body, 
either  oblong  or  oval  in  outline.  This  conclusion, 
however,  was  soon  found  to  be  untenable,  because 
the  two  (supposed)  tributary  bodies  gradually  de- 
creased in  size  until  they  entirely  disappeared. 
Galileo  writing  to  his  friend,  Welser,  in  December 
1612,  thus  expressed  himself: — 

"  What  is  to  be  said  concerning  so  strange  a 
metamorphosis  ?  Are  the  two  lesser  stars  con- 
sumed after  the  manner  of  the  solar  spots  ? 
Have  they  vanished  or  suddenly  fled  ?  Has  Sat- 
urn, perhaps,  devoured  his  own  children  ?  Or 
were  the  appearances  indeed  illusion  or  fraud, 
with  which  the  glasses  have  so  long  deceived  me, 
as  well  as  many  others  to  whom  I  have  shewn 
them  ?  Now,  perhaps,  is  the  time. come  to  revive 
the  well-nigh  withered  hopes  of  those  who,  guided 
by  more  profound  contemplations,  have  discovered 
the  fallacy  of  the  new  observations,  and  demon- 
strated the  utter  impossibility  of  their  existence. 
I  do  not  know  what  to  say  in  a  case  so  surprising, 


126        THE  STORY  OF  THE  SOLAR  SYSTEM. 


SATURN.  127 

so  unlocked  for,  and  so  novel.  The  shortness  of 
the  time,  the  unexpected  nature  of  the  event,  the 
weakness  of  my  understanding,  and  the  fear  of 
being  mistaken  have  greatly  confounded  me." 

Galileo  seems  to  have  become  so  out  of  heart 
in  consequence  of  the  difficulty  of  determining 
what  these  changes  really  meant,  that  he  gave  up 
altogether  observing  Saturn.  In  the  course  of 
time,  but  by  very  gradual  steps,  astronomers 
came  to  realise  what  the  facts  were.  The  next 
idea  that  was  broached,  was  that  the  planet  con- 
sisted of  simply  one  central  ball,  and  that  the 
excrescences  which  Galileo  had  been  puzzled  by 
were  merely  handles  as  they  were  called,  (ansce) 
projecting  like  the  handles,  say  of  a  soup  tureen, 
though  why  they  should  vary  in  size  at  stated 
intervals  remained  as  great  a  mystery  as  ever. 
It  was  not  until  about  1656  that  the  true  ex- 
planation was  arrived  at  by  a  Dutchman,  named 
Christopher  Huygens.  It  was  the  fashion  in 
those  days  for  scientific  men  to  intimate  to  the 
world  discoveries  which  they  had  made  by  resort 
to  mysterious  anagrams,  which  served  in  some 
degree  the  purpose  which  in  the  present  day  is 
served  by  the  law  regulating  copyright  or  patent 
rights.  Accordingly  Huygens  published  the  fol- 
lowing singular  memorandum  : — 

aaaaaaa  cccc  d  eeeee  g  h  i 
iiiiii  1111  mm  nnnnnnnnn  oo 
oo  pp  q  rr  s  ttttt  uuuuu. 

These  letters  arranged  in  their  proper  order 
furnish  the  following  Latin  sentence  : — 

Annulo  cingitur,  tenui,  piano,  nusquam  cohaerente, 
ad  eclipticam  inclinato ;  which  Latin  sentence  be- 
comes in  the  English  tongue  : — 

"  [The  planet]  is  surrounded  by  a  slender  flat 

9 


128         THE  STORY  OF   THE   SOLAR   SYSTEM. 

ring  inclined  to  the  ecliptic,  but  which  nowhere 
touches  [the  body  of  the  planet.]  " 

Huygen's  discovery  was  not  a  mere  piece  of 
guesswork,  for  he  spent  several  years  carefully 
observing  the  alterations  of  form  which  Saturn 
underwent,  before  he  came  to  the  conclusion  that 
it  was  only  the  existence  of  a  ring  surrounding 
the  planet  which  would  explain  the  various  ob- 
served changes. 

It  was  by  way  of  guarding  himself  from  being 
robbed  of  the  fruits  of  his  discovery  whilst  he 
was  accumulating  the  necessary  proof  of  its  truth, 
that  he  buried  his  thoughts  in  the  logogriph  or 
anagram  just  quoted.  Having  arrived  at  the 
conclusion  which  he  did,  he  thought  himself  suf- 
ficiently sure  of  his  facts  to  predict  that  in  July 
or  August  1671,  the  planet  would  again  appear 
round,  the  ring  becoming  invisible.  This  surmise 
proved  practically  correct,  in  so  far,  that  in  May 
1671,  or  within  2  months  of  the  time  predicted 
by  Huygens,  Cassini  saw  the  planet  as  a  simple 
ball  unaccompanied  by  any  ring.  ^ 

This  is  a  convenient  place  at  which  to  offer  a 
brief  explanation  of  the  changes  of  appearance  as 
regards  the  ball  and  rings  which  Saturn  under- 
goes. These  changes  depend  jointly  on  Saturn's 
motion  in  its  orbit  round  the  Sun,  and  on  the 
corresponding  motion  of  the  Earth  in  its  orbit. 
Neither  Saturn  nor  the  Earth  revolve  round  the 
Sun  exactly  in  the  ecliptic,  and  this  want  of  coin- 
cidence results  in  the  fact,  that  twice  in  the  29^ 
years  occupied  by  Saturn  in  journeying  round  the 
Sun,  the  plane  of  its  ring  is  seen  edgeways  by  us 
on  the  Earth  ;  whilst  at  two  other  periods  inter- 
mediate but  equi-distant  the  ring  is  seen  opened 
out  to  the  widest  possible  extent ;  that  is,  so  far 


SATURN. 


129 


130         THE  STORY  OF  THE  SOLAR  SYSTEM. 

as  we  on  the  Earth  can  by  any  possibility  have  a 
chance  of  seeing  it. 

The  appearances  presented  by  the  rings  when 
undergoing  the  transformations  to  which  they 
are  subject,  will  be  readily  understood  by  an  in- 
spection of  the  annexed  engravings.  Fig.  17, 
indicates  the  actual  appearances  in  the  years 
specified,  and  these  years  may  be  considered  as 
carried  forward  and  brought  up  to  date  by  sub- 
stituting 1877  for  1848,  1885  for  1855,  1891  for 
1862,  and  1898  for  1869. 

Adverting  to  fig.  16,  it  will  suffice  to  remark 
that  the  two  central  phases  of  the  rings,  opened 
wide,  are  to  be  deemed  co-related,  or  indeed 
identical  in  a  geometrical  sense  (so  to  speak) 
the  difference  being  that  one  of  them  is  to  be 
deemed  to  show  the  northern  side  of  the  ring 
(which  is  now  in  view  and  will  continue  in  view 
till  1907)  whilst  the  other  represents  the  southern 
side,  which  was  in  view  from  1877  till  1891.  The 
foregoing  is  a  brief  statement  of  the  general  prin- 
ciple involved  in  the  changes  which  take  place, 
but  the  motions  of  the  two  planets  introduce  cer- 
tain technical  complications  into  the  details  which 
would  be  seen  by  an  observer  using  a  large  tele- 
scope;  with  these,  however,  the  ordinary  reader 
will  not  care  to  concern  himself,  and  need  not 
do  so. 

A  great  deal  might  be  said  with  respect  to  the 
rings  treated  descriptively.  I  will  now  mention 
a  few  matters  of  general  interest.  Huygens  re- 
garded the  appendage  to  Saturn,  whose  existence 
he  established,  to  be  a  single  ring,  but  as  far  back 
as  1675,  Cassini  determined  that  Huygen's  single 
ring  was  really  made  up  of  two,  one  lying  inside 
the  other.  Cassini  in  this  conclusion  outstepped 


SATURN.  131 

not  only  all  the  observers  of  his  own  century,  but 
those  of  the  succeeding  century,  for  Sir  W.  Her- 
schel  even  100  years  after  Cassini,  was  for  a  long 
time  unable  to  satisfy  himself,  even  with  his 
superior  telescopes,  that  the  black  streaks  seen 
in  the  ring  by  Cassini,  and  regarded  by  him  as 
indicative  of  a  severance  of  the  ring  into  two 
parts,  really  implied  a  severance.  It  is  now,  how- 
ever, accepted  as  a  fact  that  not  only  are  the 
rings  which  are  known  as  A  and  B  absolutely 
distinct,  but  that  A  also  is  itself  certainly  duplex, 
that  is,  that  it  certainly  consists  of  two  independ- 
ent rings.  In  addition  to  this  many  competent 
observers  armed  with  powerful  telescopes  have 
obtained  traces  of  other  sub-divisions,  both  in  A 
and  B;  and  though  there  is  some  want  of  har- 
mony in  the  details,  as  stated  by  the  different 
observers,  yet  undoubtedly  we  must  speak  of 
Saturn's  rings  collectively  as  forming  a  multiple 
system. 

What  the  rings  are  is  a  highly  debatable 
point,  but  the  preponderating  idea  is  that  they 
are  not  what  they  appear  to  be,  namely  solid 
masses  of  matter,  but  are  swarms  of  independent 
fragments  of  matter.  Yet  "  fragment  "  is  n9t  the 
best  word  to  use,  because  it  implies  that  some- 
thing has  been  broken  up  to  make  the  fragments. 
Rather,  perhaps,  we  should  say  with  Professor 
Young,  that  the  rings  are  4<  composed  of  a  swarm 
of  separate  particles,  each  a  little  independent 
moon  pursuing  its  own  path  around  the  planet. 
The  idea  was  suggested  long  ago,  by  J.  Cassini  in 
1715,  and  by  Wright  in  1750,  but  was  lost  sight  of 
until  Bond  revived  it  in  connection  with  his  dis- 
covery of  the  dusky  ring.  Professor  Benjamin 
Pierce  soon  afterwards  demonstrated  that  the 


132          THE   STORY  OF  THE   SOLAR   SYSTEM. 

rings  could  not  be  continuous  solids;  and  Clerk 
Maxwell  finally  showed  that  they  can  be  neither 
solid  nor  liquid  sheets,  but  that  all  the  known 
conditions  would  be  answered  by  supposing  them 
to  consist  of  a  flock  of  separate  and  independent 
bodies,  moving  in  orbits  nearly  circular,  and  in 
one  plane — in  fact,  a  swarm  of  meteors." 

The  thickness  of  the  rings  seen  edgeways  has 
been  variously  estimated.  Sir  J.  Herschel  sug- 
gested 250  miles  as  an  outside  limit,  which  G.  P. 
Bond  reduced  to  40  miles.  It  is  generally  con- 
sidered, however,  that  100  miles  is  probably  not 
far  from  the  truth.  Young  has  pointed  out  that 
if  a  model  of  them  were  constructed  on  the  scale 
of  i  inch  to  represent  10,000  miles,  so  that  the 
outer  ring  of  such  a  model  would  be  nearly  17 
inches  in  diameter,  then  the  thickness  of  the  ring 
would  be  represented  by  that  of  an  ordinary  sheet 
of  writing  paper. 

Considered  as  a  system,  the  rings  are  distinctly 
more  luminous  than  the  planet,  and  of  the  two 
bright  rings,  the  inner  one  is  brighter  than  the 
outer  one;  and  the  inner  one  is  less  bright  at  its 
inner  edge  than  elsewhere.  It  is  also  to  be 
noticed  that  when  seen  edgeways  just  about  the 
time  of  the  Saturnian  equinoxes,  when  the  Sun  is 
shifting  over  from  one  side  of  the  ring  to  the 
other,  and  the  ring  is  dwindling  down  to  a  narrow 
streak,  its  edges  (forming  the  ans<z  as  they  are 
termed)  do  not  disappear  and  reappear  at  the 
same  time,  and  are  not  always  of  the  same  ap- 
parent extent.  One  ansa,  indeed,  is  sometimes 
visible  without  the  other,  and  most  commonly  it 
is  the  Eastern  one  that  is  missing.  To  what 
causes  these  various  peculiarities  are  due  is  un- 
known. 


SATURN.  133 

Many  physical  peculiarities  have  been  either 
noticed  or  suspected  with  reference  to  the  bright 
rings.  For  instance,  on  comparing  one  with 
another,  some  persons  have  thought  that  their 
surfaces  are  convex,  and  that  they  do  not  lie  in 
the  same  plane.  The  existence  of  mountains  on 
their  surface  has  more  than  once  been  suspected. 
Again,  it  has  been  fancied  that  they  are  sur- 
rounded by  an  extensive  atmosphere.  It  seems 
hardly  likely  that  the  rings  would  have  an  atmos- 
phere and  not  the  ball  (or  vice  versa),  and,  there- 
fore, no  wonder  that  we  have  no  observations 
which  countenance  the  idea  that  the  ball  does 
really  possess  an  atmosphere.  This,  indeed, 
seems  to  flow  from  Trouvelot's  observation,  that 
the  ball  is  less  luminous  at  its  circumference  than 
at  its  centre. 

The  circumstances  of  ring  C,  otherwise  called 
the  "  Dusky "  or  "  Crape  "  ring  are  as  curious 
historically,  as  they  are  mysterious  physically. 
In  1838,  Galle  of  Breslau,  noticed  what  he 
thought  to  be  a  gradual  shading  off  of  the  in- 
terior bright  ring  towards  the  ball.  Though  he 
published  a  statement  of  what  he  saw,  the  matter 
seems  to  have  attracted  little  or  no  notice.  In 
1850,  G.  P.  Bond  in  America  perceived  something 
luminous  between  the  ring  and  the  ball,  and  after 
repeated  observations  in  concert  with  his  father, 
came  to  the  conclusion  that  the  luminous  appear- 
ance which  he  saw,  was  neither  less  nor  more 
than  an  independent  and  imperfectly  illuminated 
ring  lying  within  the  old  rings  and  concentric 
with  them.  Before,  however,  tidings  of  Bond's 
discovery  reached  England,  but  a  few  days  after 
the  discovery  in  point  of  actual  date,  Dawes  sud- 
denly noticed  one  evening  as  Bond  had  done,  a 


134         THE   STORY  OF  THE   SOLAR  SYSTEM. 

luminous  shading  within  the  bright  rings,  which 
he  was  not  long  in  finding  out  to  be  in  reality  a 
complete  ring,  except  so  far  that  a  portion  of  it 
was  of  course  hidden  from  view  behind  the  ball. 
He,  and  O.  Struve  likewise,  noticed  that  this  new 
Dusky  Ring  was  occasionally  to  be  seen  divided 
into  two  or  more  rings.  The  Dusky  Ring  is 
transparent,  though  this  fact  was  not  ascertained 
until  1852,  or  two  years  after  Bond's  discovery  of 
the  ring. 

The  Dusky  Ring  is  now  recognised  as  a  per- 
manent feature  of  Saturn,  but  how  far  it  used  to 
be  permanent,  or  how  long  it  has  been  so,  is  a 
matter  wrapped  in  doubt.  Recorded  observations 
by  Picard  in  1673,  and  by  Hadley  in  1723,  made 
of  course  with  telescopes  infinitely  less  powerful 
than  those  of  the  present  day,  seem  to  suggest 
that  both  the  observers  named  saw  the  Dusky 
Ring,  without,  however,  being  able  to  form  a 
clear  conception  that  it  was  a  ring.  It  is  strange 
that  during  the  long  period  from  1723  to  1838, 
no  one — not  even  Sir  W.  Herschel,  with  his  vari- 
ous telescopes — should  have  obtained  or  at  least 
have  recorded  any  suspicion  of  its  existence. 
There  is,  however,  direct  evidence  that  the  Dusky 
Ring  is  wider  and  less  faint  than  formerly.  This 
was  directly  confirmed  by  Carpenter  in  1863,  who 
says  he  saw  it  "nearly  as  bright* as  the  illumi- 
nated ring,  so  much  so,  that  it  might  easily  have 
been  mistaken  for  a  part  of  it."  In  1883,  David- 
son found  a  marked  difference  in  the  brilliancy  of 
the  two  ends  (ansce)  of  the  ring. 

In  1889  Barnard  was  fortunate  enough  to  ob- 
serve an  eclipse  of  one  of  Saturn's  satellites  by 
the  ring,  but  the  eclipse,  that  is  the  concealment 
of  the  satellite,  was  only  effected  when  it  passed 


SATURN.  135 

behind  the  bright  rings;  the  dusky  ring  did  not 
obliterate  it,  and  hence  there  was  obtained  a  con- 
clusive proof  of  the  transparency  of  the  dusky 
ring.  Barnard  further  concluded  from  his  obser- 
vations that  there  was  no  separating  space  or 
division  between  the  inner  bright  ring  and  the 
dusky  ring,  as  has  frequently  been  represented  in 
drawings.  This  transparency  of  the  Dusky  Ring, 
as  a  matter  of  fact,  is  therefore  undoubted ;  yet 
what  are  we  to  consider  to  be  the  meaning  of  an 
observation  by  Wray  in  1861,  that  whilst  looking 
at  the  dusky  ring  edgeways  the  impression  was 
conveyed  to  his  eye  that  that  ring  was  very  much 
thicker  than  the  bright  rings  ? 

A  very  interesting  question  which  has  been 
much  discussed  has  reference  to  the  stability  of 
the  rings.  It  is  generally  agreed  that  the  con- 
stituent particles  of  the  rings  must  be  in  motion 
round  the  primary  or  their  equilibrium  could  not 
be  maintained :  almost  equally  certain  is  it,  and 
for  the  like  reason,  that  the  rings  cannot  be  solid. 
Of  actual  change  in  the  rings  as  regards  their 
dimensions,  we  have  no  satisfactory  proof,  though 
authorities  differ  on  the  point,  some  thinking  that 
the  rings  are  expanding  inwards,  so  that  ulti- 
mately they  will  come  into  contact  with  the  ball, 
whilst  others  consider  no  proof  whatever  of  such 
change  can  be  obtained  from  any  of  the  observa- 
tions yet  made  in  the  way  of  measurements. 

We  must  now  proceed  to  consider  the  satel- 
lites of  Saturn.  These  are  8  in  number,  7  of 
which  move  in  orbits  whose  planes  coincide 
nearly  with  the  planet's  equator,  whilst  the  re- 
maining one  is  inclined  about  12°  thereto.  One 
consequence  of  this  coincidence  in  the  planes  of 
these  satellites,  which,  it  should  be  stated,  are  the 


136         THE  STORY  OF  THE  SOLAR  SYSTEM. 

7  innermost,  is  that  they  are  always  visible  to  the 
inhabitants  of  both  hemispheres  when  they  are 
not  actually  undergoing  eclipse  in  the  shadow  of 
Saturn.  The  satellites  are  of  various  sizes,  and 
succeed  one  another  in  the  following  order,  reck- 
oning from  the  nearest,  outwards : — Mimas,  En- 
celadus,  Tethys,  Dione,  Rhea,  Titan,  Hyperion 
and  lapetus.  Any  good  2-inch  telescope  will 
show  Titan ;  a  3-inch  will  sometimes  show  lape- 
tus;  a  4-inch  will  show  lapetus  well,  together 
with  Rhea  and  Dione,  but  hardly  Tethys,;  all  the 
others  require  large  telescopes.  If  Saturn  has 
any  inhabitants  at  all  constituted  like  ourselves, 
which  is  highly  improbable,  they  will  have  a 
chance  of  seeing  celestial  phenomena  of  the 
greatest  interest.  What  with  the  rings  surround- 
ing the  planet  and  8  moons  in  constant  motion, 
there  will  be  an  endless  succession  of  astronom- 
ical sights  for  them  to  study.  The  amount  of 
light  received  from  the  Sun  cannot  be  much — 
barely  y^o-th  of  what  the  earth  receives.  The 
ring  and  satellites  will  therefore  be  useful  as  sup- 
plementary sources  of  light ;  yet  the  satellites 
will  not  furnish  much,  for  it  has  been  calculated 
that  the  surface  of  the  sky  occupied  by  all  the 
satellites  put  together  would  to  a  dweller  on 
Saturn  only  amount  to  6  times  the  area  of  the 
sky  covered  by  our  Moon ;  whilst  the  intrinsic 
brightness  of  all  put  together  would  be  no  more 
than  -^g-th  part  of  the  light  which  we  receive  from 
our  Moon. 

The  only  physical  fact  worth  noting  here  in 
connection  with  the  satellites  concerns   lapetus. 
Cassini  two  centuries  ago  with  his  indifferent  tel- 
.escopes  thought  he  had  ascertained  that  this  satel- 
lite was  subject  to  considerable  variations  of  bril- 


SATURN.  137 

liancy.  Sir  W.  Herschel  confirmed  Cassini  as  to 
this.  He  found  that  it  was  much  less  brilliant 
when  traversing  the  eastern  half  of  its  orbit  than 
at  other  times.  Two  conclusions  have  been  drawn 
from  this  fact.  One  is  that  the  satellite  rotates 


FIG.  18. — Saturn  with  the  shadow  of  Titan  on  it,  March  u, 
1892  (Terby). 

once  on  its  axis  in  the  same  time  that  it  performs 
one  revolution  round  its  primary  ;  and  that  there 
are  portions  of  its  surface  which  are  almost  en- 
tirely incapable  of  reflecting  the  rays  of  the  Sun. 
This  last  named  supposition  may  perhaps  be  well 
founded,  but  the  former  needs  more  proof  than  is 
as  yet  forthcoming.  lapetus  on  the  whole  may 
be  said  to  shine  as  a  star  of  the  9th  magnitude. 
To  this  it  may  be  added  that  Titan  is  of  the  8th 
magnitude,  but  all  the  others  much  smaller. 

Saturn  revolves  round  the  Sun  in  a  little  under 
29^  years  at  a  mean  distance  of  886  millions  of 
miles.  Its  apparent  diameter  varies  between  15" 
and  2o;/;  its  true  diameter  may  be  put  at  75,000 
miles.  The  flattening  of  the  poles,  or  "  polar 
compression  "  as  it  is  called,  is  greater  than  that 


138          THE   STORY   OF  THE   SOLAR   SYSTEM. 

of  any  other  planet,  but  is  usually  less  noticeable 
than  in  the  case  of  Jupiter,  because  the  ring  is 
apt  to  distract  the  eye,  except  when  near  the  edge- 
ways phase.  The  compression  may  be  taken  at  £. 


CHAPTER   XI. 

URANUS. 

To  the  Ancients  Saturn  was  the  outermost 
planet  of  the  System,  nothing  beyond  it  being 
known.  Nor  indeed  was  it  to  be  assumed  that 
any  more  could  possibly  exist,  because  Mercury, 
Venus,  the  Earth,  Mars,  Jupiter,  and  Saturn,  with 
the  Sun,  made  7  celestial  bodies  of  prime  impor- 
tance; and  7  was  the  number  of  perfection;  and 
there  was  thus  provided  one  celestial  body  to  give 
a  name  to  each  of  the  days  of  the  week. 

But  Science  is  not  sentimental ;  and  when  men 
of  Science  come  upon  what  looks  like  a  discovery 
they  do  their  best  to  bring  their  discovery  to  a 
successful  issue,  however  much  people's  prejudices 
may  seem  to  stand  in  the  way  at  the  moment. 

On  a  certain  evening  in  March,  1781,  Sir  Wil- 
liam Herschel,  then  gradually  coming  into  notice 
as  a  practical  astronomer,  was  engaged  in  looking 
at  different  fields  of  stars  in  the  constellation 
Gemini  when  he  lighted  on  one  which  at  once  at- 
tracted his  special  attention.  Altering  his  eye- 
piece, and  substituting  a  higher  magnifying  power 
he  found  the  apparent  size  of  the  mysterious  ob- 
ject enlarged,  which  conclusively  proved  that  it 
was  not  a  star;  for  it  is  a  well-known  optical 
property  of  all  stars  that  whatever  be  the  size  of 


URANUS.  139 

telescope  employed  on  them,  and  however  high 
the  magnifying  power  no  definite  disc  of  light  can 
be  obtained  when  in  focus.  Herschel's  new  find, 
therefore,  was  plainly  not  a  star,  and  no  idea  hav- 
ing in  those  days  come  into  men's  minds  of  there 
being  any  new  planets  awaiting  discovery,  he  an- 
nounced as  a  matter  of  course  that  he  had  found 
a  new  comet,  so  soon  as  he  ascertained  that  the 
new  body  was  in  motion.  The  announcement 
was  not  made  to  the  Royal  Society  till  April  26, 
more  than  six  weeks  after  the  date  of  the  actual 
discovery,  an  indication,  by  the  way,  of  the  dila- 
tory circulation  of  news  a  hundred  years  ago. 
The  supposed  comet  was  observed  by  Maskelyne, 
the  Astronomer  Royal,  four  days  after  Herschel  had 
first  seen  it,  and  Maskelyne  seems  to  have  at  once 
got  the  idea  into  his  head  that  he  was  looking  at 
a  planet  and  not  at  a  comet.  As  soon  as  possible 
after  the  discovery  of  a  new  comet  the  practice 
of  astronomers  is  to  endeavour  to  determine  what 
is  the  shape  of  the  orbit  which  it  is  pursuing.  All 
attempts  to  carry  out  this  in  the  case  of  Herschel's 
supposed  new  comet  proved  abortive,  because  it 
was  found  impossible  to  harmonise,  except  for  a 
short  period  of  time,  the  movements  of  the  new 
body  with  the  form  of  curve  usually  affected  by 
most  comets,  namely,  the  parabola.  It  is  true,  as 
we  shall  see  later  on  in  speaking  of  comets,  that 
a  certain  number  of  those  bodies  do  revolve  in 
the  closed  curve  known  as  the  ellipse,  but  it  is 
usual  to  calculate  the  parabolic  form  first  of  all, 
because  it  is  the  easier  to  calculate ;  and  to  perse- 
vere with  it  until  it  plainly  appears  that  the  parab- 
ola will  not  fit  in  with  the  observed  movements 
of  the  new  object.  This  practice  was  carried  out 
in  the  case  of  Herschel's  new  body,  and  it  was 


140         THE   STORY  OF  THE  SOLAR  SYSTEM. 

eventually  found  that  not  only  was  its  orbit  not 
parabolic;  that  not  only  was  its  orbit  not  an  elon- 
gated ellipse  of  the  kind  affected  by  comets;  but 
that  it  was  nearly  a  circle,  and  as  the  body  itself 
showed  a  defined  disc  the  conclusion  was  inevita- 
ble :  it  was  in  real  truth  a  new  planet.  It  has  not 
taken  long  to  write  this  statement,  and  it  will 
take  still  less  time  for  the  reader  to  read  what 
has  been  written,  but  the  result  just  mentioned 
occupied  the  attention  of  astronomers  many 
months  in  working  out,  step  by  step,  in  such  a 
way  as  to  make  sure  that  no*  mistake  had  been 
made. 

When  it  was  once  clearly  determined  that 
Herschel  had  added  a  new  planet  to  the  list  of 
known  planets  it  became  an  interesting  matter 
of  inquiry  to  find  out  whether  it  had  ever  been 
seen  before;  and  to  settle  the  name  it  should  bear. 
A  little  research  soon  showed  that  the  new  planet 
had  been  seen  and  recorded  as  a  fixed  star  by 
various  observers  on  20  previous  occasions,  be- 
ginning as  far  back  as  Dec.  13,  1690,  when  Flam- 
stead  registered  at  Greenwich  as  a  star.  These 
various  observations,  spread  over  a  period  of  91 
years,  and  all  recorded  by  observers  of  skill  and 
eminence  materially  helped  astronomers  in  their 
efforts  to  calculate  accurately  the  shape  and  na- 
ture of  the  new  planet's  orbit.  One  observer,  a 
Frenchman  named  Le  Monnier,  saw  the  planet  no 
less  than  12  times  between  1750  and  1771,  and  if 
he  had  had  (which  it  is  known  he  had  not)  an  or- 
derly and  methodical  mind,  the  glory  of  this  dis- 
covery would  have  been  lost  to  England  and 
obtained  by  France.  Arago  has  left  it  on  record 
that  he  was  once  shown  one  of  these  chance  ob- 
servations of  Uranus,  which  had  been  recorded 


URANUS.  141 

by  Le  Monnier  on  an  old  paper  bag  in  which  hair 
powder  had  been  sold  by  a  perfumer. 

A  long  d  scussion  took  place  on  the  question 
of  a  name  for  the  new  planet.  Bode's  suggestion 
of  "  Uranus  "  is  now  in  universal  use,  but  it  is 
within  the  recollection  of  many  persons  living 
that  this  planet  bore  sometimes  the  name  of  the 
"  Georgium  Sidus  "  and  sometimes  the  name  of 
"  Herschel."  The  former  designation  was  pro- 
posed by  Herschel  himself  in  compliment  to  his 
sovereign  and  patron  George  III.  of  England; 
whilst  a  French  astronomer  suggested  the  latter 
name.  However,  neither  of  these  appellations 
was  acceptable  to  the  astronomers  of  the  Con- 
tinent, who  declared  in  favour  of  a  mythological 
name,  though  it  was  a  long  time  before  they 
agreed  to  accept  Bode's  "  Uranus."  The  symbol 
commonly  used  to  represent  the  planet  is  formed 
of  Herschel's  initial  with  a  little  circle  added 
below,  though  the  Germans  employ  something 
else,  "  made  in  Germany,"  to  quote  a  too  familiar 
phrase. 

The  visible  disc  of  Uranus  is  so  small  that 
none  but  telescopes  of  the  very  largest  size  can 
make  anything  of  it.  A  few  sentences  therefore 
will  dispose  of  this  part  of  the  subject.  The  disc 
is  usually  bluish  in  tinge,  and  most  observers  who 
look  at  it  consider  it  uniformly  bright,  but  there 
is  satisfactory  testimony  to  the  effect  that  under 
the  most  favourable  circumstances  of  instrument 
and  atmosphere  two  or  more  belts,  not  unlike  the 
belts  of  Jupiter,  may  be  traced.  From  the  posi- 
tion in  which  these  belts  have  been  seen  it  is  in- 
ferred that  the  satellites  of  Uranus  (presently  to 
be  mentioned)  are  unusually  much  inclined  to  the 
planet's  equator,  and  revolve  in  a  retrograde  direc- 


142         THE  STORY  OF  THE  SOLAR  SYSTEM. 

tion,  contrary  to  what  is  the  ordinary  rule  of  the 
planets  and  satellites.  It  is  assumed  as  the  basis 
of  these  ideas,  (and  by  analogy  it  is  reasonable 
to  do  this)  that  the  belts  are  practically  parallel 
to  the  planet's  equator,  and  at  right  angles  to  the 
planet's  axis  of  rotation.  To  speak  of  the  planet's 
axis  of  rotation  is,  in  one  sense,  another  assump- 
tion, because  available  observations  can  scarcely 
be  said  to  enable  us  to  demonstrate  that  the  planet 
does  rotate  on  its  axis,  yet  we  can  have  no  moral 
doubt  about  it.  Taylor  has  suggested,  grounds 
for  the  opinion  that  "  there  can  be  very  little 
doubt  that  Uranus  is  to  a  very  large  extent  self- 
luminous,  and  that  we  do  not  see  it  wholly  by  re- 
flected light."  To  this  Gore  adds  the  idea  that 
there  is  "  strong  evidence  in  favour  of  the  exist- 
ence of  intrinsic  heat  in  the  planet." 

Uranus  is  attended  by  several  satellites.  It 
was  once  thought  that  there  were  eight,  of  which 
six  were  due  to  Sir  W.  Herschel,  the  other  two 
being  of  modern  discovery.  Astronomers  are, 
however,  now  agreed  that  no  more  than  four 
satellites  can  justly  be  recognised  as  known  to 
exist,  and  they  are  so  minute  in  size  that  only  the 
very  largest  telescopes  will  show  them;  and  there- 
fore our  knowledge  of  them  is  extremely  limited. 
Sir  W.  Herschel's  idea  that  he  had  seen  six  satel- 
lites appears  to  have  resulted  from  his  having  on 
some  occasions  mistaken  some  very  small  stars 
for  satellites.  Two  only  of  his  six  are  thought 
to  have  been  real  satellites.  The  other  two  rec- 
ognised satellites  were  found  both  in  1847,  one  by 
Lassell,  and  the  other  by  O.  Struve. 

Uranus  revolves  round  the  Sun  in  rather  more 
than  84  years,  at  a  mean  distance  of  1781  millions 
of  miles.  Its  apparent  diameter,  seen  from  the 


NEPTUNE.  143 

Earth,  does  not  vary  much  from  3^,  which  corre- 
sponds to  about  31,000  miles.  It  has  been  calcu- 
lated that  the  light  received  from  the  Sun  by 
Uranus  would  be  about  the  amount  furnished  by 
300  full  Moons  seen  by  us  on  the  Earth,  though 
another  authority  increases  this  to  1670  full 
Moons.  From  Uranus  Saturn  can  be  seen,  and 
perhaps  Jupiter,  both  as  inferior  planets,  just  as 
we  see  Venus  and  Mercury  ;  but  all  the  other 
inner  planets,  including  Mars  and  the  Earth, 
would  be  hopelessly  lost  to  view,  because  per- 
petually too  close  to  the  Sun.  Possibly,  how- 
ever, they  might,  on  rare  occasions,  be  seen  in 
transit  across  the  Sun's  disc.  Neptune,  of  course, 
would  be  visible  and  be  the  only  superior  planet. 
The  Sun  itself  would  appear  to  an  observer  on 
Uranus  as  a  very  bright  star,  with  a  disc  of  i  J'  of 
arc  in  diameter. 


CHAPTER   XII. 

NEPTUNE. 

WE  now  come  to  the  best  known  planet  of 
the  solar  system,  reckoning  outwards  from  the 
Sun,  and  though  this  planet  itself,  as  an  object  to 
look  at,  has  no  particular  interest  for  the  general 
public,  yet  the  history  of  its  discovery  is  a  matter 
of  extreme  interest.  Moreover,  it  is  very  closely 
mixed  up  with  the  history  of  the  planet  Uranus, 
which  has  just  been  described.  After  Uranus 
had  become  fully  recognised  as  a  regular  member 
of  the  solar  system,  a  French  astronomer  named 
Alexis  Bouvard  set  himself  the  task  of  exhaust- 


144         THE   STORY  OF  THE   SOLAR   SYSTEM. 

ively  considering  the  movements  of  Uranus  with 
a  view  of  determining  its  orbit  with  the  utmost 
possible  exactness.  His  available  materials 
ranged  themselves  in  two  groups: — the  modern 
observations  between  1781  and  1820,  and  the 
early  observations  of  Flamsteed,  Bradley,  Mayer, 
and  Le  Monnier,  extending  from  1690  to  1771. 
Bouvard  found  in  substance  that  he  could  frame 
an  orbit  which  would  fit  in  with  each  group  of 
observations,  but  that  he  could  not  obtain  an 
orbit  which  would  reconcile  both  sets  of  observa- 
tions during  the  130  years  over  which  they  jointly 
extended.  He  therefore  rashly  came  to  the  con- 
clusion that  the  earlier  observations,  having  been 
made  when  methods  and  instruments  were  alike 
relatively  imperfect,  were  probably  inaccurate  or 
otherwise  untrustworthy,  and  had  better  be  re- 
jected. This  seemed  for  awhile  to  solve  the  diffi- 
culty, and  results  which  he  published  in  1821  rep- 
resented with  all  reasonable  accuracy  the  then 
movements  of  the  planet.  A  very  few  years,  how- 
ever, sufficed  to  reveal  discordances  between  ob- 
servation and  theory,  so  marked  and  regular  as 
to  make  it  perfectly  clear  that  it  was  not  Bou- 
vard's  work  which  was  faulty  but  that  Uranus 
itself  had  gone  astray  through  the  operation  of 
definite  but  as  yet  unknown  causes.  What  these 
causes  were  could  only  be  a  matter  of  surmise 
based  upon  the  evident  fact  that  there  was  some 
source  of  disturbance  which  was  evidently  throw- 
ing Uranus  out  of  its  proper  place  and  regular 
course.  First  one  and  then  another  astronomer 
gave  attention  and  thought  to  the  matter,  and 
eventually  the  conclusion  was  arrived  at  that 
there  existed,  more  remote  from  the  Sun  than 
Uranus,  an  undiscovered  planet  which  was  able 


NEPTUNE.  145 

to  make  its  influence  felt  by  deranging  the  move- 
ments of  Uranus  in  its  ordinary  journey  round 
the  Sun  every  84  years.  This  conclusion  on  the 
part  of  astronomers  becoming  known,  a  young 
Cambridge  student,  then  at  St.  John's  College, 
John  Couch  Adams  by  name,  resolved,  in  July 
1841,  to  take  up  the  subject,  though  it  was  not 
until  1843  tnat  ne  actually  did  so.  The  problem 
to  be  solved  was  to  suggest  the  precise  place  in 
the  sky  at  a  given  time  of  an  imaginary  planet 
massive  enough  to  push,  or  pull,  out  of  its  nor- 
mal place  the  planet  Uranus,  which  was  evidently 
being  pushed  at  one  time  and  pulled  at  another. 
It  would  also  be  part  of  the  problem  to  predict 
the  distance  from  the  Sun  of  the  planet  thus  im- 
agined to  exist.  Adams  worked  patiently  and 
silently  at  this  very  profound  and  difficult  prob- 
lem for  if  years  when  he  found  himself  able  to 
forward  to  Airy,  who  had  become  Astronomer 
Royal  after  being  a  Cambridge  Professor,  some 
provisional  elements  of  an  imaginary  planet  of  a 
size,  at  a  distance,  and  in  a  position  to  meet  the 
circumstances.  It  is  greatly  to  be  regretted,  on 
more  grounds  than  one,  that  Airy  did  nothing  but 
pigeon-hole  Adams's  papers.  Had  he  done  what 
might  have  been,  and  probably  was,  expected, 
that  is,  had  he  made  them  public,  or  better  still 
had  he  made  telescopic  use  of  them,  a  long  and 
unpleasant  international  controversy  would  have 
been  avoided,  and  Adams  would  not  have  been 
robbed  in  part  of  the  well-deserved  fruits  of  his 
protracted  labours. 

We  must  now  turn  to  consider  something  that 
was  happening  in  France.  In  the  summer  of 
1845,  Just  before  Adams  had  finished  his  work, 
and  one  and  a  half  years  after  he  commenced  it 


146         THE  STORY  OF  THE  SOLAR  SYSTEM. 

a  young  Frenchman,  who  afterwards  rose  to 
great  eminence,  U.  J.  J.  Le  Verrier,  turned  his 
attention  to  the  movements  of  Uranus  with  a 
view  of  ascertaining  the  cause  of  their  recognised 
irregularity.  In  November  1845  he  made  public 
the  conclusion  that  those  irregularities  did  not 
exclusively  depend  upon  Jupiter  or  Saturn.  He 
followed  this  up  in  June  1846  by  a  second  memoir 
to  prove  that  an  unknown  exterior  planet  was  the 
cause  of  all  the  trouble,  and  he  assigned  evidence 
as  to  its  position  very  much  as  Adams  -had  do*ne 
8  months  previously.  Airy  on  receiving  a  copy 
of  Le  Verrier's  memoir  seems  so  far,  at  last,  to 
have  been  roused  that  he  took  the  trouble  to 
compare  Le  Verrier's  conclusions  with  those  of 
Adams  so  long  in  his  possession  neglected.  Find- 
ing that  a  remarkably  close  accord  existed  be- 
tween the  conclusions  of  the  two  men,  he  came  to 
realise  that  both  must  be  of  value,  and  he  wrote 
a  fortnight  later  to  suggest  to  Professor  Challis 
the  desirability  of  his  instituting  a  search  for  the 
suspected  planet.  Challis  began  within  two  days, 
but  was  handicapped  by  not  having  in  his  posses- 
sion any  map  of  the  stars  in  the  neighbourhood 
suggested  as  the  locale  of  the  planet.  He  lost  no 
time  however  in  making  such  a  map,  but,  of 
course,  the  doing  so  caused  an  appreciable  delay, 
and  it  was  not  until  September  29,  1846,  that  he 
found  an  object  which  excited  his  suspicions  and 
eventually  proved  to  be  the  planet  sought  for.  It 
was  subsequently  ascertained  that  the  planet  had 
been  recorded  as  a  star  on  August  4  and  12,  and 
that  the  star  of  August  12  was  missing  from  the 
zone  observed  on  July  30.  The  discovery  of  the 
planet  was  therefore  just  missed  on  August  12 
because  the  results  of  each  evening's  work  were 


NEPTUNE.  147 

not   adequately  compared   with  what   had   gone 
before. 

Meanwhile  things  had  not  been  standing  still 
in  France.  In  August  1846,  Le  Verrier  published 
a  third  memoir  intended  to  develope  information 
respecting  the  probable  position  of  the  planet  in 
the  heavens.  In  September  23  a  summary  of  this 
third  memoir  was  received  by  Encke  at  Berlin, 
accompanied  by  the  request  that  he  would  co- 
operate instrumentally  in  the  search  for  it.  Encke 
at  once  directed  two  of  his  assistants  named 
D'Arrest  and  Galle  to  do  this,  a^nd  they  were  for- 
tunately well  circumstanced  for  the  task.  Unlike 
Challis,  who,  as  we  have  seen,  could  do  nothing 
until  he  had  made  a  map  for  himself,  the  Berlin 
observers  had  one  ready  to  hand,  which  by  good 
chance  had  just  been  published  by  the  Berlin 
Academy  for  the  part  of  the  heavens  which  both 
Adams  and  Le  Verrier  assigned  as  the  probable 
locality  in  which  the  anxiously  desired  planet 
would  be  found.  Galle  called  out  the  visible 
stars  one  by  one  whilst  D'Arrest  checked  them 
by  the  map,  and  suddenly  he  came  upon  an  un- 
marked object  which  at  the  moment  looked  like 
an  8th  magnitude  star.  The  following  night 
showed  that  the  suspicious  object  was  in  motion, 
and  it  was  soon  ascertained  to  be  the  trans-Ura- 
nian  planet  which  was  being  searched  for.  The 
discovery  when  announced  excited  the  liveliest 
interest  all  over  the  world.  It  did  more  ;  it  cre- 
ated a  bitter  feeling  of  resentment  on  the  part  of 
French  astronomers  that  the  laurels  claimed  by 
them  should  have  been  also  claimed  in  an  equal 
share  by  a  young  and  unknown  Englishman,  and 
accordingly  the  old  cry  of  " perfide  Albion  "  arose 
on  a.ll  sides.  I  have  been  particular  in  stating 


148        THE  STORY  OF  THE  SOLAR  SYSTEM. 

the  various  dates  which  belong  to  this  narrative, 
in  order  to  make  as  clear  as  possible  the  facts  of 
the  case.  This  is  even  now  necessary,  because 
though  the  astronomers  of  England  and  Ger- 
many are  willing  to  give  Adams  and  Le  Verrier 
each  their  fair  share  of  this  great  discovery,  the 
same  impartial  spirit  is  not  to  be  found  in  France, 
for  nothing  is  more  common,  even  in  the  present 
day,  in  looking  at  French  books  of  astronomy, 
than  to  find  Adams's  name  either  glossed  over  or 
absolutely  suppressed  altogether  when  the  planet 
Neptune  is  under  discussion. 

How  remarkable  a  discovery  this  was,  will 
perhaps  be  realized,  when  it  is  stated  that  Adams 
was  only  2j°  out  in  assigning  the  position  of  the 
new  planet,  whilst  Le  Verrier  was  even  nearer, 
being  barely  i°  out. 

We  know  practically  nothing  respecting  the 
physical  appearance  of  Neptune,  owing  to  its  im- 
mense distance  from  us,  and  for  the  like  reason 
the  Neptunian  astronomers,  if  there  are  any,  will 
know  absolutely  nothing  about  the  Earth  ;  in- 
deed, their  knowledge  of  the  Solar  System  will 
be  restricted  to  Uranus,  Saturn,  and  the  Sun. 
Even  the  Sun  will  only  have  an  apparent  diameter 
of  about  i'  of  arc,  and,  therefore,  will  only  seem 
to  be  a  very  bright  star,  yielding  light  equal  in 
amount,  according  to  Zollner,  to  about  700  full 
moons.  There  is  one  satellite  belonging  to  Nep- 
tune, and  as  this  has  been  calculated  to  exhibit  a 
disc  10°  in  diameter,  a  certain  amount  of  light 
will  no  doubt  be  afforded  by  it  especially  if,  as  is 
not  unlikely,  Neptune  is  itself  possessed, of  some 
inherent  luminosity  independently  of  the  Sun. 

The  fact  that  Neptune  seems  destitute  of  vis- 
ible spots  or  belts,  results  in  our  being  igno- 


NEPTUNE.  149 

rant  of  the  period  of  its  axial  rotation,  though  it 
should  be  stated  that  in  1883,  Maxwell  Hall  in 
Jamaica,  observed  periodical  fluctuations  in  its 
light,  which  he  thought  implied  that  the  planet 
rotated  on  its  axis  in  rather  less  than  8  hours. 
Several  observers  thought  20  or  30  years  ago, 
that  they  had  noticed  indications  of  Neptune 
being  surrounded  by  a  ring  like  Saturn's  ring, 
but  the  evidence  as  to  this  is  very  inconclusive. 
It  is  quite  certain  that  none  but  the  very  largest 
telescopes  in  the  world  would  show  any  such 
appendage,  and  this  planet  seems  to  have  been 
neglected  of  late  years,  by  the  possessors  of  such 
telescopes.  Moreover,  if  a  ring  existed  it  would 
only  open  out  to  its  full  extent  once  in  every  82 
years,  being  the  half  of  the  period  of  the  planet's 
revolution  round  the  Sun  (just  as  Saturn's  ring 
only  opens  out  to  the  fullest  extent  every  14^ 
years),  so  that,  obviously,  supposing  suspicions 
of  a  ring  dating  back  30  or  40  years  were  well 
founded,  it  might  well  be  that  another  30  or  40 
years  might  need  to  elapse,  before  astronomers 
would  be  in  a  position  to  see  their  suspicions 
revive. 

Neptune  revolves  round  the  Sun  in  164^ 
years,  at  a  mean  distance  of  2791  millions  of 
miles.  Its  apparent  diameter  scarcely  varies 
from  2f".  Its  true  diameter  is  about  37,000 
miles.  No  compression  of  the  Poles  is  percepti- 
ble. Its  one  satellite  revolves  round  Neptune  in 
5f  days,  and  in  a  retrograde  direction,  at  a  mean 
distance  of  223,000  miles,  and  shines  as  a  star  of 
the  i4th  magnitude.  This  is  a  peculiarity  which 
it  only  shares  with  the  satellites  of  Uranus,  so  far 
as  it  regards  the  planetary  members  of  the  Solar 
System,  though  there  are^many  retrograde  Comets, 


150         THE   STORY   OF  .THE   SOLAR  SYSTEM. 

The  question  has  often  been  mooted,  whether 
there  exists,  and  belonging  to  the  Solar  System, 
a  planet  farther. off  than  Neptune.  There  does 
seem  some  evidence  of  this,  as  we  shall  better 
understand,  when  we  come  to  the  subject  of  long- 
period  Comets,  though  it  cannot  be  said  that  much 
progress  has  yet  been  made  in  arriving  at  a  solu- 
tion of  the  problem. 

Unless  there  does  exist  a  trans-Neptunian 
planet,  a  Neptunian  astronomer  will  know  very 
little  about  planets,  for  Uranus  and  Saturn  will 
alone  be  visible  to  him.  Both  will  of  course  be 
what  we  call  "  inferior  planets,"  and  under  the 
best  of  circumstances  will  cut  a  poor  figure  in  the 
Neptunian  sky. 


CHAPTER   XIII. 

COMETS. 

I  SUPPOSE  that  it  is  the  experience  of  all  those 
who  happen  to  be  in  any  sense,  however  humble, 
specialists  in  a  certain  branch  of  science,  that 
from  time  to  time,  they  are  beset  with  questions 
on  the  part  of  their  friends  respecting  those  par- 
ticular matters  which  it  is  known  that  they  have 
specially  studied.  There  is  no  fault  to  be  found 
with  this  thirst  for  information,  always  supposing 
that  it  is  kept  within  due  bounds;  but  my  motive 
for  alluding  to  it  here,  is  to  see  whether  any  well- 
marked  conclusion  can  be  drawn  from  it,  within  • 
my  own  knowledge  as  regards  astronomical  facts 
or  events.  Now  in  the  case  of  the  science  of 
astronomy  (for  which  in  this  connection  I,  for  the 


COMETS.  151 

moment,  will  venture  to  speak),  there  is  certainly 
no  one  department  which  so  unfailingly,  at  all 
times  and  in  all  places,  seems  to  evoke  such  pop- 
ular sympathy  and  interest  as  the  department 
which  deals  with  Comets. 

Sun-spots  may  come  and  go  ;  bright  planets 
may  shine  more  brightly;  the  Sun  or  Moon  may 
be  obscured  by  eclipses ;  temporary  stars  may 
burst  forth, — all  these  things  are  within  the  ken 
of  the  general  public  by  means  of  newspapers  or 
almanacs,  but  it  is  a  comet  which  evokes  more 
questionings  and  conversations  than  all  the  other 
matters  just  referred  to  put  together.  When  a 
new  and  bright  comet  appears,  or  even  when  any 
comet  not  very  bright  gets  talked  about,  the  old 
question  is  still  fresh  and  verdant — "  Is  there  any 
danger  to  the  Earth  to  be  apprehended  from  col- 
lision with  a  Comet?"  followed  by  "  What  is  a 
Comet  ? "  "  What  is  it  made  of  ?  "  "  Has  it  ever 
appeared  before  ? "  "  Will  it  come  back  again  ?  " 
and  so  on.  Questions  in  this  strain  have  more 
often  than  I  can  tell  of  been  put  to  me.  They 
seem  the  stock  questions  of  all  who  will  con- 
descend to  replace  for  five  minutes  in  the  day 
the  newest  novel  or  the  pending  parliamentary 
election. 

It  may  be  taken  as  a  fact  (though  in  no  proper 
sense  a  rule)  that  a  bright  and  conspicuous  comet 
-  comes  about  once  in  10  years,  and  a  very  remark- 
able comet  every  30  years.  Thus  we  have  had 
during  the  present  century  bright  comets  in  1811, 
1825,  1835,  1843,  1858,  1861,  1874  and  1882,  whereof 
those  of  1811,  1843,  and  1858  were  specially  cele- 
brated. Tested  .then  by  either  standard  of  words 
"bright  and  conspicuous,"  or  "specially  cele- 
brated," it  may  be  affirmed  that  a  good  comet  is 


152         THE  STORY  OF  THE  SOLAR  SYSTEM. 

now  due,  so  let  us  prepare  for  it  by  getting  up  the 
subject  in  advance. 

I  will  not  attempt  to  answer  in  regular  order 
or  in  any  set  form  the  questions  which  I  have 
just  mentioned  as  being  stock  questions,  but  they 
will  be  answered  in  substance  as  we  go  along. 
There  is  one  matter  in  connection  with  comets 
which  has  deeply  impressed  itself  upon  the  public 
mind,  and  that  is  the  presence  or  absence  of  a 
"  tail."  It  is  not  too  much  to  say  that  the  gener- 
ality of  people  regard  the  tail  of  a  comet  as  the 
comet;  and  that  though  an  object  may  be  a  true 
comet  from  an  astronomer's  point  of  view,  yet  if 
it  has  no  tail  its  claims  go  for  nought  with  the 
mass  of  mankind.  We  have  here  probably  a 
remnant  of  ancient  thought,  especially  of  that 
line  of  thought  which  in  bygone  times  associated 
Comets  universally  with  the  idea  that  they  were 
especially  sent  to  be  portents  of  national  disasters 
of  one  kind  or  another.  This  is  brought  out  by 
numberless  ancient  authors,  and  by  none  more 
forcibly  than  Shakespeare.  Hence  we  have  such 
passages  as  the  following  in  Julius  Ccesar  (Act  ii., 
sc.  2) : — 

"  When  beggars  die  there  are  no  comets  seen, 
The  Heavens  themselves  blaze  forth  the  death  of  princes." 

In  Henry  VI.  (Part  I.,  act  i.,  sc.  i)  we  find  the 
well-known  passage  : — 

"  Comets  importing  change  of  times  and  states 
Brandish  your  crystal  tresses  in  the  sky, 
And  with  them  scourge  the  bad  revolting  stars 
That  have  consented  unto  Henry's  death." 

There  are  in  point  of  fact  two  distinct  ideas 
evolved  here :  (i)  that  comets  are  prophetic  of 
evil,  and  (2)  stars  potential  for  evil, 


COMETS.  153 

There  is  another  passage  in  Henry  VI.  (Part 
I.,  act  iii.,  sc.  3)  even  more  pronounced: — 

"Now  shine  it  like  a  Comet  of  revenge, 
A  prophet  to  the  fall  of  all  our  foes." 

Again  ;  in  Hamlet  (Act  i.,  sc.  i)  we  find : — 

"  As  stars  with  trains  of  fire,  and  dews  of  blood, 
Disasters  in  the  Sun." 

Once  more ;  in  the  Taming  of  the  Shrew  (Act 
iii.,  sc.  2)  we  have  the  more  general,  but  still  em- 
phatic enough,  idea  expressed  by  the  simple  words 
of  reference  to — 

"  Some  Comet  or  unusual  prodigy." 

Shakespeare  may  be  said  to  have  lived  at  the 
epoch  when  astrology  was  in  high  favour,  and  it 
may  be  that  he  only  gave  utterance  to  the  current 
opinion^  prevalent  among  all  classes  in  those  still 
somewhat  "  Dark  Ages  "  (so  called).  This,  how- 
ever, can  hardly  be  said  of  the  author  of  my  next 
quotation — John  Milton  {Paradise  Lost,  bk.  II.)  : — 

"  Satan  stood 

Unterrified,  and  like  a  Comet  burned, 
That  fires  the  length  of  Ophiuchus  huge 
In  th'  Arctic  sky,  and  from  its  horrid  hair 
Shakes  pestilence  and  war." 

Jumping  over  a  century  we  find  the  ancient 
theory  still  in  vogue,  or  Thomson  (Seasons ^  Sum- 
mer) would  never  have  written  : — 

"  Amid  the  radiant  orbs 
That  more  than  deck,  that  animate  the  sky, 
The  life-infusing  suns  of  other  worlds  ; 
Lo  !   from  the  dread  immensity  of  space, 
Returning  with  accelerated  course, 
The  rushing  comet  to  the  sun  descends ; 
And,  as  he  sinks  below  the  shading  earth, 
With  awful  train  projected  o'er  the  heavens. 
The  guilty  nations  tremble." 


154      ,  THE  STORY  OF  THE   SOLAR   SYSTEM. 


Even  Napoleon  I.  had  servile  flatterers  who,  as 
late  as  1808,  tried  to  extract  astrological  influence 
out  of 'a  comet  by  way  of  bolstering  up  "  Old 
Bony."  But  enough  of  poetry  and  fiction,  let  us 
hasten  back  to  prosaic  fact. 

Comets  as  objects  to  look  at  may  be  classed 
under  three  forms,  though  the  same  comet  may 
undergo  such  changes  as  will  at  different  epochs 
in  its  career  cause  it  to  put  on  each  variety  of 
form  in  succession.  Thus  the  comet  of  1825  seen 
during  that  year  as  a  brilliant  naked-eye  object, 
after  being  lost  in  the  sun's 
rays,  was  again  found  on 
April  2,  1826  by  Pons. 
Lamentable  were  his  cries 
at  the  miserable  plight  it 
was  in.  He  described  it 
as  totally  destroyed  : 
without  tail,  beard,  coma 
or  nucleus,  a  mere  spectre. 
The  simplest  form  of 
comet  is  a  mere  nebulous 
mass,  almost  always  cir- 
cular, or  perhaps  a  little 
oval,  in  outline.  It  may  maintain  this  appearance 
throughout  its  visibility  ;  or,  growing  brighter 
may  become  a  comet  of  the  second  class,  with  a 
central  condensation,  which  developing  becomes 
a  "nucleus  "  or  head.  It  may  retain  this  feature 
for  the  rest  of  its  career,  or  may  pass  into  the 
third  class  and  throw  out  a  "coma"  or  beard, 
which  will  perhaps  develop  into  a  tail  or  tails. 
Doing  this  it  will  not  unfrequently  grow  bright 
enough  and  large  enough  to  become  visible  to 
the  naked  eye.  In  exceptional  cases  the  nucleus 
will  become  as  bright  as  a  2nd  or  even  ist  mag- 


FIG.  19. — Telescope  Comet 
with  a  nucleus. 


COMETS.  155 

nitude  star,  and  the  tail  may  acquire  a  length  of 
several  or  many  degrees.  In  the  last  named  case 
of  all  the  comet  becomes, /#r  excellence  according 
to  the  popular  sentiment,  "  a  comet."  It  will  now 
be  readily  inferred  that  the  astronomer  in  his  ob- 
servatory has  to  do  with  many  comets  which  the 
public  at  large  never  hear  of,  or  if  they  do  hear 
of,  treat  with  contempt,  because  they  are  destitute 
of  tails. 

The  tails  of  comets  exhibit  very  great  varieties 
not  only  of  size  but  of  form ;  some  are  long  and 


FIG.  20. — Wells's  Comet  of  1882,  seen  in  full  daylight  near  the 
Sun  on  Sept.  18. 

slender;  some  are  long  and  much  spread  out 
towards  their  ends,  like  quill  pens,  for  instance; 
some  are  short  and  stumpy,  mere  tufts  or  excres- 
cences rather  than  tails.  Not  unfrequently  a  tail 
seems  to  consist  of  two  parallel  rays  with  no 
cometary  matter,  or  it  may  be  only  a  very  slight 
amount  of  cometary  matter  traceable  in  the  in- 


156         THE  STORY  OF  THE  SOLAR  SYSTEM. 


FIG.  21. — Quenisset's  Comet,  July  9,  1893  (Quenisset). 


COMETS.  157 

terspace;  some  have  one  main  tail  consisting  of 
a  pair  of  rays  such  as  just  described,  together 
with  one  or  more  subsidiary  or  off-shoot  tails. 
The  comet  of  1825  had  five  tails  and  the  comet 
of  1744  had  six  tails.  It  might  be  inferred  from 
all  this  that  the  tails  of  comets  are  so  exceedingly 
irregular,  uncertain  and  casual  as  to  be  amenable 
to  no  laws.  This  was  long  considered  to  be  the 
case;  but  a  Russian  observer  named  Bredichin, 
as  the  result  of  much  study  and  research,  has 
arrived  at  the  conclusion  that  all  comet  tails  may 
be  brought  under  one  or  other  of  three  types; 
and  that  each  type  is  indicative  of  certain  dis- 
tinct differences  of  origin  and  condition  which  he 
considers  himself  able  to  define.  The  first  type 
comprises  tails  which  are  long  and  straight; 
"they  are  formed  "  (to  quote  Young's  statement 
of  Bredichin's  views)  "  of  matter  upon  which  the 
Sun's  repulsive  action  is  from  twelve  to  fifteen 
times  as  great  as  the  gravitational  attraction,  so 
that  the  particles  leave  the  comet  with  a  relative 
velocity  of  at  least  four  or  five  miles  a  second  ; 
and  this  velocity  is  continually  increased  as  they 
recede,  until  at  last  it  becomes  enormous,  the 
particles  travelling  several  millions  of  miles  in  a 
day.  The  straight  rays  which  are  seen  in  the 
figure  of  the  tail  of  Donati's  Comet,  tangential  to 
the  tail,  are  streamers  of  this  first  type;  as  also 
was  the  enormous  tail  of  the  comet  of  1861. 
The  second  type  is  the  curved  plume-like  train, 
like  the  principal  tail  of  Donati's  Comet.  In  this 
type  the  repulsive  force  varies  from  2.2  times 
gravity  (for  the  particles  on  the  convex,  edge  of 
the  tail)  to  half  that  amount  for  those  which  form 
the  inner  edge.  This  is  by  far  the  most  common 
type  of  cometary  train.  A  few  comets  show  tails 


158         THE  STORY  OF  THE   SOLAR  SYSTEM. 

of  the  third  type — short,  stubby,  brushes  violently 
curved,  and  due  to  matter  of  which  the  repulsive 
force  is  only  a  fraction  of  gravity — from  TVto  £." 

Bredichin  wishes  it  to  be  inferred  that  the 
tails  of  the  ist  type  are  probably  composed  of 
hydrogen  ;  those  of  the  2nd  type  of  some  hydro- 
carbon gas;  and  those  of  the  3rd  of  the  vapour 
of  iron,  probably  with  some  admixture  of  sodium 
and  other  substances.  Bredichin,  as  a  reason  for 
these  conclusions,  supposes  that  the  force  which 
generates  the  tails  of  comets  is  a  repulsive  force, 
with  a  surface  action  the  same  for  equal  surfaces 
of  any  kind  of  matter ;  the  effective  accelerating 
force  therefore  measured  by  the  velocity  which 
it  would  produce  would  depend  upon  the  ratio 
of  surface  to  mass  in  the  particles  acted  upon, 
and  so,  in  his  view,  should  be  inversely  propor- 
tional to  their  molecular  weights.  Now  it  hap- 
pens that  the  molecular  weights  of  hydrogen,  of 
hydro-carbon  gases,  and  of  the  vapour  of  iron 
bear  to  each  other  just  about  the  required  pro- 
portion. 

I  am  here  stating  the  views  and  opinions  of 
others  without  definitely  professing  to  be  satis- 
fied with  them,  but  as  they  have  met  with  some 
acceptance,'  it  is  proper  to  chronicle  them,  though 
we  know  nothing  of  the  nature  of  the  repulsive 
force  here  talked  about.  It  might  be  electric,  it 
might  be  anything.  The  spectroscope  certainly 
lends  some  countenance  to  Bredichin's  views,  but 
we  need  far  more  knowledge  and  study  of  comets 
before  we  shall  be  justly  entitled  to  dogmatise  on 
the  subject. 

This  has  been  rather  a  digression.  I  go  back 
now  to  prosaic  matters  of  fact,  of  which  a  vast 
and  interesting  array  present  themselves  for  con- 


COMETS. 


159 


sideration    in    connection    with   comets.     Let   us 
consider  a  little  in  detail  what  they  are,  to  look 


FIG.  22.— Holmes's  Comet,  Nov.  9,  1892  (Denning). 

at.  We  have  seen  that  a  well-developed  comet 
of  the  normal  type  usually  comprises  a  nucleus, 
a  head  or  coma,  and  a  tail.  Comets  which  have 


FIG.  23. — Holmes's  Comet,  Nov.  16,  1892  (Denning). 

no  tails  generally  exhibit   heads  of   very   simple 
structure;  and  if  there  is  a  nucleus,  the  nucleus 
is  little  else  than  a  stellar  point  of  light.     But  in 
ii 


160         THE   STORY   OF    THE   SOLAR   SYSTEM. 


the  case  of  the  larger  comets,  which  are  almosi 
or  quite  visible  to  the  naked  eye,  the  head  ofter 
exhibits  a  very  complex  structure,  which  in  not  £ 
few  cases  seems  to  convey  very  definite  indica 
tions  of  the  operations  going  on  at  the  time 
Figs.  22  and  23  may  be  taken  as  samples  of  c. 
complex  cometary  head,  though  no  two  comets- 
resemble  one  another  exactly  in  details.  Fig.  24 
forcibly  conveys  the  idea  that  we  are  looking  at 
a  process  of  development  analogous  to  an  uprush 
of  water  from  a  fountain,  or  perhaps  I  might  bet- 
ter say,  from  a  burst  waterpipe.  There  is  a  dis- 
tinct idea  of  a  jet.  This  self-same  idea,  in  another 
form,  presents  itself  in  the  case  of  those  comets 
which  exhibit  what  astronomers  are  in  the  habit 
of  calling  "  luminous  envelopes."  The  jet  in  this 
case  is  not  strictly  a  jet 
because  it  is  not  a  con- 
tinuous outflow,  or  over- 
flow, of  matter  ;  the  idea 
rather  suggests  itself  of  an 
intermittent  overflow  re- 
sulting in  accumulated  lay- 
ers, or  strata,  of  matter 
becoming  visible.  But 
with  this  we  come  to  a 
standstill;  we  cannot  tell 
FIG.  24.— Comet  in.  of  1862,  where  the  matter  comes 

on  Aug  22,  showing  jet  of    from    and   still    less    where 
luminous  matter  (Challis). 

it  goes    to;    we  can  only 

record  what  our  eyes,  assisted  by  telescopes,  tell 
us.  There  can,  however,  I  think,  be  no  doubt 
that  the  matter  of  a  comet  becomes  displayed  to 
our  senses  as  the  result  of  a  process  of  expulsion, 
or  repulsion,  from  the  nucleus ;  and  then,  having 
become  launched  into  space,  it  comes  under  the 


COMETS.  l6l 

influence,  also  repulsive,  of  the  Sun.  All  these 
things  are  visible  facts.  As  to  causes,  we  suggest 
little,  because  we  know  so  little.  Anyone  who 
has  seen  a  comet  and  has  watched  the  displays  of 
jets  and  luminous  envelopes,  such  as  I  have  en- 
deavoured to  set  forth,  will  realise  at  once  how 
impossible  it  is  to  describe  these  things  in  words. 
They  must  be  seen  either  in  actual  being  or  in 
picture.  Some  further  allusions  to  this  branch  of 
the  subject  may  perhaps  be  more  advantageously 
made  after  we  have  considered  the  movements 
and  orbits  of  comets. 

There  is  often  a  slight  general  resemblance 
between  a  planet  and  a  comet,  as  regards  the 
path  which  each  class  of  body  pursues.  Probably 
the  least  reflective  person  likely  to  be  following 
me  here  understands  the  bare  fact,  that  all  the 
planets  revolve  round  the  Sun,  and  are  held  to 
defined  orbits  by  the  Sun's  influence,  or  attrac- 
tion, as  it  is  called.  Perhaps,  it  is  not  equally 

i  realised,  that  in  a  somewhat  similar,  but  not  quite 
the  same  way,  comets  are  influenced  and  con- 
trolled by  the  Sun. 

Comets  must  be  considered  as  regards  their 
motions  to  be  divisible  into  two  classes  : — (i) 
Those  which  belong  to  the  Solar  System ;  and  (2) 

i  those  which  do  not.  Each  of  these  two  classes 
must  again  be  sub-divided,  if  we  would  really  ob- 
tain a  just  conception  of  how  things  stand. 

By  the  Comets  which  belong  to  the  Sun,  I 
mean  those  which  revolve  round  the  Sun  in 

i  closed  orbits ;  *  and  are,  or  may  be,  seen  again 
and  again  at  recurring  intervals.  There  are  2 


- 


*  The  circle  and  the  ellipse  are  what  are  called  "  closed  " 
•ves. 


162         THE  STORY  OF  THE  SOLAR  SYSTEM. 

or  3  dozen  comets  which  present  themselves  to 
our  gaze  at  stated  intervals,  varying  from  about 
3  to  70  years.  There  are  again  other  comets 
which  without  any  doubt  (mathematically)  are 
revolving  round  the  Sun  in  closed  orbits,  but  in 
orbits  so  large  and  with  periods  of  revolution  so 
long  (often  many  centuries),  that  though  they 
will  return  again  to  the  sight  of  the  inhabitants 
of  the  earth  some  day,  yet  no  second  return  hav- 
ing been  actually  recorded,  the  astronomer's  pre- 
diction that  they  will  return,  remains  at  present 
a  prediction  based  on  mathematics  but  nothing 
more. 

There  is  another  class  of  Comet  of  which  we 
see  examples  from  time  to  time,  and  having  seen 
them  once  shall  never  see  again.  This  is  be- 
cause these  Comets  move  in  orbits  which  are  not 
closed,  and  which  are  known  as  parabolic  or  hy- 
perbolic orbits  respectively,  because  derived  from 
those  sections  of  a  cone  which  are  called  the 
Parabola  and  the  Hyperbola.  It  must  be  under- 
stood that  what  I  am  now  referring  to  is  purely 
a  matter  of  orbit,  and  that  no  relationship  sub- 
sists between  the  size  and  physical  features  of  a 
Comet  and  the  path  it  pursues  in  space.  The 
only  sort  of  reservation,  perhaps,  to  be  made  to 
this  statement  is,  that  the  comets  celebrated  for 
their  size  and  brilliancy,  are  often  found  to  be 
revolving  in  elliptic  orbits  of  great  eccentricity, 
which  means  that  their  periods  may  amount  to 
many  centuries. 

It  may  be  well  to  say  something  now  as  to 
what  is  the  ordinary  career  of  a  comet,  so  far  as 
visibility  to  us,  the  inhabitants  of  the  Earth,  is 
concerned.  Though  this  might  be  illustrated  by 
reference  to  the  history  of  many  comets,  perhaps 


COMETS.  163 

there  is  no  one  more  suitable  for  the  purpose 
than  Donati's  Comet  of  1858.  In  former  times, 
when  telescopes  were  few  or  non-existent,  bril- 
liant comets  often  appeared  very  suddenly,  just  as 
a  carriage  or  a  man  does,  as  you  turn  the  corner 
of  a  street.  Such  things  even  happen  still :  for 
instance,  the  great  comet  of  1861  burst  upon  us 
all  at  once  at  a  day's  notice.  Usually,  however, 
now  in  consequence  of  the  large  size  of  the  tele- 
scopes in  use,  and  the  great  number  of  observ- 
ers who  are  incessantly  on  the  watch,  comets  are 
discovered  when  they  are  very  small,  because  re- 
mote both  from  the  Earth  and  Sun,  and  many 
weeks,  or  even  months,  it  may  be,  before  they 
shine  forth  in  their  ultimate  splendour.  Now,  let 
us  see  how  these  statements  are  supported  by  the 
history  of  Donati's  comet  in  1858.  On  June  2  in 
that  year,  it  was  first  seen  by  Donati  at  Florence, 
as  a  faint  nebulosity,  slowly  journeying  north- 
wards. June  passed  away,  and  July,  and  August, 
the  comet  all  the  while  remaining  invisible  to  the 
naked  eye;  that  is  to  say,  it  first  became  percep- 
tible to  the  naked  eye  on  August  29,  having  put 
forth  a  faint  tail  about  August  20.  After  the 
beginning  of  September  its  brilliancy  rapidly 
increased.  On  September  17,  the  head  equalled 
in  brightness  a  2nd  magnitude  star,  the  tail  being 
4°  long.  Passing  its  point  of  nearest  approach  to 
the  Sun  on  September  29,  it  came  nearest  to  the 
Earth  on  October  10  ;  though,  perhaps,  its  ap- 
pearance a  few  days  previously,  namely  on  Octo- 
ber 5,  is  the  thing  best  remembered  by  those  who 
saw  it,  because  it  was  on  that  night  that  the  comet 
passed  over  the  ist  magnitude  star  Arcturus. 
For  several  days  about  this  time,  the  comet  was 
an  object  of  striking  beauty  in  the  Western 


1 64         THE   STORY   OF   THE   SOLAR   SYSTEM. 

Heavens,  during  the  hours  immediately  after 
sun-set.  After  October  10,  it  rapidly  passed 
away  to  the  Southern  hemisphere,  diminishing 
in  brightness,  as  it  did  so,  because  receding  from 
the  Earth  and  the  Sun.  It  continued  its  career 
through  the  winter ;  became  invisible  to  the 
naked  eye ;  and  finally  invisible  altogether  in 
March  1859.  It  remained  in  view,  therefore,  for 
more  than  nine  months,  not  to  return  again  till 
about  the  year  3158  A.  D.,  for  its  period  of  revo- 
lution was  found  to  be  about  2000  years. 

I  have  been  particular  in  sketching  somewhat 
fully  the  history  of  this  comet  so  far  as  we  are 
concerned,  because,  as  I  have  already  said,  it  is 
typical  of  the  visible  career  of  many  comets. 
Halley's  comet  in  1835  and  1836,  went  through 
a  somewhat  similar  series  of  changes.  This 
comet — a  well-known  periodical  one  of  great 
historic  interest  and  brilliancy — may  be  com- 
mended to  the  younger  members  of  the  rising 
generation,  because  it  is  due  to  return  again 
to  these  parts  of  space  a  few  years  hence,  or  in 
1910. 

What  is  a  comet  made  of  ?  Men  of  Science 
equally  with  the  general  public  would  like  to  be 
able  to  answer  this  question,  but  they  cannot  do 
so  with  satisfactory  certainty.  A  great  many 
years  ago  Sir  John  Herschel  wrote  thus  : — "  It 
seems  impossible  to  avoid  the  following  conclu- 
sion, that  the  matter  of  the  nucleus  of  a  comet  is 
powerfully  excited  and  dilated  into  a  vaporous 
state  by  the  action  of  the  Sun's  rays  escaping  in 
streams  and  jets  at  those  points  of  its  surface 
which  oppose  the  least  resistance,  and  in  all  prob- 
ability throwing  that  surface  or  the  nucleus  it- 
self into  irregular  motions  by  its  reaction  in  the 


COMETS. 


165 


a 

o 
U 


I 


1 66         THE  STORY  OF  THE  SOLAR  SYSTEM. 

act  of  so  escaping,  and  thus  altering  its  direc- 
tion." This  passage  was  written  of  course  before 
the  spectroscope  had  been  brought  to  bear  on  the 
observations  of  comets,  but  so  far  as  Sir  John 
Herschel's  remark  implies  the  presence  of  va- 
pour, that  is  gas,  in  a  comet,  the  surmise  has 
been  amply  borne  out  by  later  discoveries.  The 
fact  that  as  a  comet  approaches  the  Sun  some 
forces,  no  doubt  of  solar  origin,  come  into  opera- 
tion to  vaporise  and  therefore  expand  the  matter 
composing  the  comet  is  sufficiently  shown  by  the 
great  developement  which  takes  place  as  we  have 
seen  in  the  tails  of  comets,  but  in  regard  to  the 
heads  of  comets  we  are  face  to  face  with  a  strange 
enigma.  Though  the  tails  expand  the  heads  con- 
tract as  the  comet  approaches  its  position  of 
greatest  proximity  to  the  Sun.  Having  passed 
this  point  the  head  expands  again.  This  curious 
circumstance,  first  pointed  out  by  Kepler  in  1618, 
has  often  been  noticed  since,  and  noticed  indeed 
not  as  the  result  of  mere  eye  impressions,  but 
after  careful  micrometrical  measurement  with  suit- 
able instruments.  I  think  the  confession  must  be 
made  that  we  are  hopelessly  ignorant  of  the  nature 
of  comet's  except  that  gases  are  largely  concerned 
in  their  constitution. 

It  seems  impossible  to  doubt  that  some  tails 
of  comets  are  hollow  cylinders  or  hollow  cones. 
Such  a  theory  would  account  for  the  fact,  so 
often  noticed,  that  single  tails  are  usually  much 
brighter  at  their  two  edges  than  at  the  centre. 
This  is  the  natural  effect  of  looking  transversely 
at  any  translucent  cylinder  of  measureable  thick- 
ness. 

It  was  long  a  moot  point  whether  comets  are 
self-luminous,  or  whether  they  shine  by  reflected 


COMETS.  167 

light;  but  it  is  now  generally  admitted  that  whilst 
a  part  of  the  light  of  a  comet  may  be  derived  by 
reflection  from  the  Sun  yet  as  a  rule  they  must 
be  regarded  as  shining  by  their  own  intrinsic  light. 

It  should  be  stated  here  by  way  of  caution 
that  the  observations  on  this  subject  are  not  so 
consistent  as  one  could  wish,  and  it  seems  neces- 
sary to  assume  that  all  comets  are  not  constituted 
alike,  and  that  therefore  what  is  true  of  one  does 
not  necessarily  apply  to  another. 

To  those  who  possess  telescopes  (not  neces- 
sarily large  ones)  opportunities  for  the  study  of 
comets  have  much  multiplied  during  the  last  few 
years,  for  we  are  now  acquainted  with  a  group  of 
small  comets  which  are  constantly  coming  into 
view  at  short  intervals  of  time.  The  comets  have 
now  become  so  numerous  that  seldom  a  year 
passes  without  one  or  more  of  them  coming  into 
view.  Whilst  that  known  as  Encke's  revolves 
round  the  Sun  in  3^  years,  Tuttle's  doing  the 
same  in  13^  years,  there  are  four  others  whose 
periods  average  about  5^  years,  5  which  average 
6J-  years,  together  with  one  of  y£  years  and  one 
of  8  years.  It  is  thus  evident  that  there  is  a  con- 
stant succession  of  these  objects  available  for 
study,  and  that  very  few  months  can  ever  elapse 
that  some  one  or  more  of  them  are  not  on  view. 
They  bear  the  names  of  the  astronomers  who  either 
discovered  them  originally,  or  who,  by  studying 
their  orbits,  discovered  their  periodicity.  The 
names  run  as  follows,  beginning  with  the  shortest 
in  period  and  ending  with  the  longest : — 


Encke's. 

Temple's  Second  (1873, 
n.) 


Winnecke's. 

Brorsen's. 

Temple's  First  (1867,  n.) 


1 68          THE   STORY   OF  THE   SOLAR   SYSTEM. 


Swift's  (1880,  v.) 
Barnard's  (1884,  11.) 
D'Arrest's. 
Finlay's. 


Wolf's  (1884,  m.) 
Faye's. 
Denning's. 
Tuttle's. 


I  cannot  stay  to  dwell  upon  either  the  history 
or  description  of  these  comets  separately,  but 
must  content  myself  by  saying  generally  t'hat 
whilst  as  a  rule  they  are  not  visible  to  the  naked 
eye,  yet  several  of  them  may  occasionally  become 
so  visible  when  they  return  to  perihelion-  under 
circumstances  which  bring  them  more  near  than 
usual  to  the  earth. 

Several  other  comets  are  on  record  which  it 
W7as  supposed  at  one  time  would  certainly  have 
been  entitled  to  a  place  in  the  above  list,  but 
three  of  them  in  particular  have,  under  very 
mysterious  circumstances,  entirely  disappeared 
from  the  Heavens. 

Chief  amongst  the  mysterious  comets  must  be 
ranked  that  which  goes  by  the  name  of  Biela. 
This  comet,  first  seen  in  1772,  was 'afterwards 
found  to  have  a  period  of  about  6f  years,  and  on 
numerous  occasions  it  reappeared  at  intervals  of 
that  length  down  to  1845,  when  the  mysterious 
part  of  its  career  seems  to  have  commenced.  In 
December  of  that  year  this  comet  threw  off  a 
fragment  of  nearly  the  same  shape  as  itself,  and 
the  two  portions  travelled  together  side  by  side 
for  four  months,  the  distance  between  the  frag- 
ments slowly  increasing.  At  the  end  of  the  four 
months  in  question  the  comet  passed  out  of  sight 
owing  to  the  distance  from  the  earth  to  which  it 
had  attained.  The  comet  returned  again  to  peri- 
helion in  1852,  remaining  visible  for  three  weeks. 
The  two  portions  of  the  comet  noticed  in  1846 


COMETS.  169 

retained  their  individuality  in  1852,  but  the  dis- 
tance between  them  had  increased  to  about  eight 
times  the  greatest  distance  noticed  in  1846.  As 
a  comet  Biela's  Comet  has  never  been  seen  since 
1852,  and  it  must  now  be  regarded  as  having  per- 
manently disappeared.  But  what  seems  to  have 
happened  is  this,  that  Biela's  Comet  has  become 


FIG.  26. — Biela's  Comet,  February  19,  1846. 

broken  up  into  a  mass  of  meteors.  On  November 
27,  1872,  and  again  in  November  1885,  when  the 
earth  in  travelling  along  its  own  orbit  reached 
a  certain  point  where  its  orbit  intersected  the 
former  orbit  of  Biela's  Comet  the  Earth  encoun- 
tered, instead  of  the  comet  which  ought  to  have 
been  there,  a  wonderful  mass  of  meteors  ;  and  it 
is  now  generally  accepted  that  these  meteors, 


1 70         THE   STORY   OF  THE   SOLAR   SYSTEM. 

which  apparently  are  keeping  more  or  less  to- 
gether as  a  fairly  compact  swarm,  are  nought  else 
than  the  disintegrated  materials  of  what  once  was 
Biela's  Comet. 

It  is  extremely  probable  that  as  time  goes  on 
we  shall  be  able  to  say  that  an  intimate  connec- 
tion subsists  between  particular  comets  which 
have  been  and  particular  meteoric  swarms.  We 
already  possess  proof  that  other  comets  which 
once  came  within  our  view  were  at  that  time  re- 
volving round  the  Sun  in  orbits  so  comparatively 
small  that  they  should  have  reappeared  at  inter- 
vals of  half-a-dozen  or  so  years,  yet  they  have 
not  reappeared.  The  question  therefore  suggests 
itself,  Have  they  been  subject  to  some  great  in- 
ternal disaster  which  has  led  to  their  disintegra- 
tion ?  It  may  be  said  without  doubt  that  this  is 
in  the  highest  degree  probable ;  but  short  of  this, 
that  is  short  of  total  disintegration  into  small 
fragments,  we  have  several  cases  on  record  of 
what  I  may,  for  the  moment,  call  ordinary  comets 
breaking  up  into  two  or  three  fragments.  For  a 
long  while  astronomers  were  naturally  loath  to 
believe  that  this  was  possible,  and  therefore  they 
discredited  the  statements  to  that  effect  which 
had  been  made.  Though  it  would  occupy  too 
much  space  to  give  the  particulars  of  these 
comets  in  full  it  may  yet  be  worth  while  just  to 
mention  the  names  of  some  of  them,  presumed  to 
be  of  short  period,  which  seemed  nevertheless  to 
have  eluded  our  grasp.  I  would  here  specially 
mention  Liais's  Comet  of  1860  and  the  second 
comet  of  1881  as  seemingly  having  undergone 
some  sort  of  disruption  akin  to  what  happened  in 
the  case  of  Biela's  Comet. 

There  is  another  group  of  periodical  comets 


COMETS.  171 

to  be  mentioned.  These  are  six  in  number  and 
seem  to  have  periods  of  70  years  or  a  little  more. 
Of  these  three  have  not  yet  given  us  the  chance 
of  seeing  them  again  ;  two  have  paid  us  a  second 
visit,  and  therefore  their  periods  are  not  open 
to  doubt;  whilst  the  most  famous  of  this  group, 
"  Halley's,"  has  been  recorded  to  have  shown 
itself  to  the  Earth  no  less  than  25  times,  begin- 
ning with  the  year  n  B.  c.  It  was  Halley's 
comet  which  shone  over  Europe  in  April  1066, 
and  was  considered  the  forerunner  of  the  con- 
quest of  England  by  William  of  Normandy.  It 
figures  in  the  famous  Bayeux  tapestry  as  a  hairy 
star  of  strange  shape. 

It  would  seem  that  there  exists  in  some  in- 
scrutable manner  a  connection  between  each  of 
the  three  great  exterior  planets  and  certain 
groups  of  comets.  In  the  case  of  Jupiter  the 
association  is  so  very  pronounced  as  long  ago  to 
have  attracted  notice ;  but  the  French  astron- 
omer, Flammarion,  has  brought  forward  some 
suggestions  that  Saturn  has  one  comet  (and  per- 
haps two)  with  which  it  is  associated  ;  Uranus, 
two  (and  perhaps  three) ;  and  Neptune,  six ; 
whilst  farther  off  than  Neptune  the  fact  that 
there  are  two  comets,  supposed  periodical,  with- 
out a  known  planet  to  run  with  them  has  inspired 
Flammarion  to  look  with  a  friendly  eye  on  the 
idea  (often  mooted)  that  outside  of  Neptune 
there  exists  another  undiscovered  planet  revolv- 
ing round  the  sun  in  a  period  of  about  300  years. 

The  Jupiter  group  of  comets  deserves  a  few 
additional  words.  There  are  certainly  nine,  and 
perhaps  twelve  comets  revolving  round  the  Sun 
in  orbits  of  such  dimensions  that  they  either 
reach  up  to  or  slightly  overreach  the  orbit  of 


172         THE   STORY  OF   THE   SOLAR  SYSTEM. 

Jupiter.  The  effect  of  this  condition  of  things  is 
that  on  occasions  Jupiter  and  each  of  the  comets 
may  come  into  such  proximity  that  the  superior 
mass  of  Jupiter  may  exercise  a  very  seriously 
disturbing  influence  over  a  flimsy  and  ethereal 
body  like  a  comet.  There  is  reason  to  suppose 
that  some  of  the  comets  now  belonging  to  the 
Jupiter  group  have  not  done  so  for  any  great 
length  of  time,  but  having  been  wandering  about, 
either  in  elliptic  orbits  of  great  extent,  or  even  in 
parabolic  orbits,  have  accidentally  come  within 
reach  of  Jupiter,  and  so  have  been,  as  it  were, 
captured  by  him.  Hence  the  origin  of  the  term, 
the  "capture  theory,"  as  applied  to  these  family 
groups  of  comets  which  I  have  just  stated  to 
exist,  each  presided  over,  as  it  were,  by  a  great 
planet.  It  may  be  that  at  some  future  time  this 
theory  will  help  us  to  a  clue  to  the  fact  that  be- 
sides the  comets  of  Lexell  of  1770,  Blainpain  of 
1819,  and  Di  Vico  of  1844,  short  period  comets 
unaccountably  missing,  there  are  several  others 
presumed  to  have  been  revolving  in  short  period 
orbits  when  discovered,  and  as  to  which  it  is  very 
strange  that  they  should  not  have  been  seen  before 
their  only  recorded  visit  to  us,  and  equally  strange 
that  they  should  never  have  been  seen  since. 

Is  there  any  reason  to  fear  the  results  of  a 
collision  between  a  comet  and  the  Earth  ?  None 
whatever.  However  vague  may  be,  and  in  a  cer- 
tain sense  must  be,  our  answer  to  the  question, 
"What  is  a  comet?"  certain  is  it  that  every 
comet  is  a  very  imponderable  body — a  sort  of 
airy  nothing,  a  mass  of  gas  or  vapour.*  At  the 


*  It    is    not  a  little  singular  that   the  Chinese  in  bygone 
centuries  have  often  alluded  to  comets  under  the  name  of 


COMETS.  173 

same  time  it  always  has  been  and  perhaps  still  is 
difficult  to  persuade  the  public  that  whatever 
might  be  the  effect  on  a  comet  if  it  were  to  strike 
the  Earth,  the  effect  on  the  Earth,  were  it  to  be 
struck  by  a  comet,  would  be  nil.  This  is  not 
altogether  a  matter  of  speculation,  for  according 
to  a  calculation  by  Hind,  on  June  30,  1861,  the 
Earth  passed  into  and  through  the  tail  of  the 
great  comet  of  that  year  at  about  two-thirds  of 
its  distance  from  the  nucleus.  Assuredly  there 
was  no  dynamical  result ;  but  it  seems,  however, 
not  unlikely  that  there  was  an  optical  result ;  at 
any  rate,  traces  of  something  of  this  sort  were 
noted.  Hind  himself,  in  Middlesex,  observed  a 
peculiar  phosphorescence  or  illumination  of  the 
sky  which  he  attributed  at  the  time  to  an  au- 
roral glare.  Lowe,  in  Nottinghamshire  confirmed 
Hind's  statement  of  the  appearance  of  the  heav- 
ens on  the  same  day.  The  sky  had  a  yellow 
auroral  glare-like  look,  and  the  Sun,  though  shin- 
ing, gave  but  feeble  light.  The  comet  was  plainly 
visible  at  7.45  p.  m.  (during  sunshine),  and  had  a 
much  more  hazy  appearance  than  on  any  subse- 
quent evening.  Lowe  adds  that  his  Vicar  had 
the  pulpit  candles  lighted  in  the  Parish  Church 
at  7  o'clock  (it  was  a  Sunday),  though  only  five 
days  had  elapsed  since  Midsummer  day,  which 
itself  proves  that  some  sensation  of  darkness  was 
felt  even  while  the  Sun  was  shining. 

So  far  as  I  remember  there  has  been  no  such 
thing  as  a  comet  panic  during  the  present  genera- 
tion, at  any  rate  in  civilised  countries,  but  it  is 
on  record  that  there  was  a  verv  considerable 


vapours;  e.g.,  the  comet  of  1618  is  recorded  as  having  been 
"  a  white  vapour  20  cubits  long." 


174         THE  STORY  OF  THE  SOLAR  SYSTEM. 

panic  in  1832  in  connection  with  the  return  of 
Biela's  Comet  in  the  winter  of  that  year.  Olbers 
as  the  result  of  a  careful  study  in  advance  of  the 
comet's  movements  found  that  the  comet's  centre 
would  pass  only  20,000  miles  within  the  Earth's 
orbit,  and  that  as  the  nebulosity  of  the  comet 
had  in  1805  been  more  than  20,000  miles  in  diam- 
eter, it  was  certain,  unless  its  dimensions  had 
diminished  in  the  27  years,  that  some  of  the 
comet's  matter  would  overlap  the  Earth's  orbit ; 
in  other  words  would  envelop  the  Earth  itself,  if 
the  Earth  happened  to  be  there.  This  conclusion 
when  it  became  public  was  quite  enough  to  create 
a  panic  and  make  people  talk  about  the  forth- 
coming destruction  of  our  globe.  It  was  nothing 
to  the  point  (in  the  public  mind)  that  astronomers 
were  able  to  predict  that  the  Earth  would  not 
reach  the  place  where  the  comet  would  cross  the 
Earth's  orbit  until  four  weeks  after  the  comet 
had  come  and  gone.  However,  we  now  know 
that  nothing  happened,  and  I  am  justified  in 
adding  that  even  if  there  had  been  contact,  Earth 
meeting  comet  face  to  face,  nothing  (serious) 
would  have  occurred  so  far  as  the  Earth  was 
concerned. 

This  seems  a  convenient  place  for  referring  to 
a  matter  which  when  it  was  first  broached  excited 
a  great  deal  of  interest,  but  about  which  one  does 
not  hear  much  now-a-days.  The  period  of  the 
small  comet  known  as  Encke's  (which,  revolving 
as  it  does  round  the  Sun  in  a  little  more  than 
three  years,  has  the  shortest  period  of  any  of  the 
periodical  comets)  was  found  many  years  ago  to 
be  diminishing  at  each  successive  return.  That 
is  to  say,  it  always  attained  its  nearest  distance 
from  the  Sun  at  each  apparition  2^  hours  sooner 


COMETS.  175 

than  it  ought  to  have  done.  In  order  to  account 
for  this  gradual  diminution  in  the  comet's  period 
Encke  conjectured  the  existence  of  a  thin  ethereal 
medium  sufficiently  dense  to  affect  a  light  flimsy 
body  like  a  comet,  but  incapable  of  obstructing  a 
planet.  It  has  been  remarked  by  Hind  that  "  this 
contraction  of  the  orbit  must  be  continually  pro- 
gressing, if  we  suppose  the  existence  of  such  a 
medium;  and  we  are  naturally  led  to  inquire, 
What  will  be  the  final  consequence  of  this  resist- 
ance ?  Though  the  catastrophe  may  be  averted 
for  many  ages  by  the  powerful  attraction  of  the 
larger  planets,  especially  Jupiter,  will  not  the 
comet  be  at  last  precipitated  on  the  Sun  ?  The 
question  is  full  of  interest,  though  altogether 
open  to  conjecture." 

Astronomers  are  not  altogether  agreed  as  to 
the  propriety  of  this  explanation.  One  argument 
against  it  is  that  w\\h  perhaps  one  exception  none 
of  the  other  short-period  comets  (all  of  them 
small  and  presumably  deficient  in  density)  seem 
affected  as  Encke's  is.  On  the  other  hand  Sir 
John  Herschel  favoured  the  explanation  just 
given,  as  also  does  Hind  who  is  the  highest  living 
authority  on  comets.  A  German  mathematician, 
Von  Asten,  who  devoted  immense  labour  to  the 
study  of  the  orbit  of  Encke's  Comet,  thought 
there  should  be  no  hesitation  in  accepting  the 
idea  of  a  resisting  medium,  subject  to  the  limita- 
tion that  it  does  not  extend  beyond  the  orbit 
of  Mercury.  Von  Asten's  allusion  to  Mercury 
touches  a  subject  which  belongs  more  directly  to 
the  question  of  Mercury's  orbit  and  to  that  other 
very  interesting  question,  "  Are  there  any  planets, 
not  at  present  known,  revolving  round  the  Sun 
within  the  orbit  of  Mercury." 
12 


176         THE  STORY  OF  THE   SOLAR  SYSTEM. 

Which  is  the  largest  and  most  magnificent 
comet  recorded  in  history  ?  It  is  virtually  impos- 
sible to  answer  this  question,  because  of  the  ex- 
travagant and  inflated  language  made  use  of  by 
ancient  and  medieval  (I  had  almost  added,  and 
modern)  writers.  There  is  no  doubt  that  the 
comet  of  1680,  studied  by  Sir  I.  Newton,  the  tail 
of  which  was  curved,  and  from  70°  to  90°  long, 
must  have  been  one  of  the  finest  on  record,  as  it 
was  also  the  one  which  came  nearest  to  the  Sun, 
for  it  almost  grazed  the  Sun's  surface. 

The  comet  of  1744,  visible  as  it  was  in  broad 
daylight,  was,  no  doubt,  the  finest  comet  of  the 
i8th  century,  though  in  size  it  has  been  surpassed  ; 
yet  its  six  tails  must  have  made  it  a  most  remark- 
able object.  So  far  as  the  ipth  century  is  con- 
cerned, our  choice  lies  between  the  comets  of 
1811,  1843,  1858,  and  1861.  The  comet  of  1811 
is  spoken  of  by  Hind  as  "  perhaps  the  most  famous 
of  modern  times.  Independently  of  its  great 
magnitude,  the  position  of  the  orbit  and  epoch  of 
perihelion  passage,  were  such  as  to  render  it  a 
very  splendid  circumpolar  object  for  some 
months."  The  tail  as  regards  its  length  was  not 
so  very  remarkable,  for  at  its  best,  in  October 
1811,  it  was  only  about  25°  long,  its  breadth,  how- 
ever, was  very  considerable  ;  at  one  time  6°,  the 
real  length  of  the  tail,  about  the  middle  of  Octo- 
ber, was  more  than  100,000,000  of  miles,  and  its 
breadth  about  15,000,000  of  miles.  The  visibility 
of  this  comet  was  coincident  with  those  events 
which  proved  to  be  the  turning-point  in  the  career 
of  Napoleon  I.,  and  there  were  not  wanting  those 
who  regarded  the  comet  as  a  presage  of  his  disas- 
trous failure  in  Russia.  Owing  to  the  long  period 
(17  months),  during  which  this  comet  was  visible, 


COMETS, 


177 


FIG.  27.— The  Great  Comet  of  1811, 


178         THE   STORY  OF  THE  SOLAR  SYSTEM. 

it  was  possible  to  determine  its  orbit  with  unusual 
precision.  Argelander  found  its  period  to  be  3065 
years,  with  no  greater  uncertainty  than  43  years. 
The  great  dimensions  of  its  orbit  will  be  realised 
when  it  is  stated  that  this  comet  recedes  from  the 
Sun  to  a  distance  of  14  times  that  of  the  planet 
Neptune. 

Donati's  comet  of  1858,  has  already  received 
a  good  deal  of  notice  at  my  hands,  but  the  ques- 
tion remains,  what  are  its  claims,  to  be  regarded 
as  the  comet  of  the  century,  compared  with  that 
of  1843?  It  is  not  a  little  strange  that  though 
there  must  have  been  many  persons  who  saw  both, 
yet  it  was  only  quite  recently  that  I  came  across, 
for  the  first  time,  a  description  of  both  these 
comets  from  the  same  pen.  It  ought,  however, 
to  be  mentioned  by  way  of  explanation,  that  the 
inhabitants  of  Europe  only  saw  the  comet  of  1843, 
when  its  brilliancy  and  the  extent  of  its  tail  had 
materially  diminished,  about  a  fortnight  after  it 
was  at  its  best. 

The  description  of  these  two  comets  to  which 
I  have  alluded,  will  be  found  in  General  J.  A. 
Ewart's  "  Story  of  a  Soldier  s  Life"  published  in 
1881.  Writing  first  of  all  of  the  comet  of  1843, 
General  Ewart  says : — 

"  It  was  during  our  passage  from  the  Cape  of 
Good  Hope  to  the  Equator,  and  when  not  far  from 
St.  Helena,  that  we  first  came  in  sight  of  the  great 
comet  of  1843.  1°  tne  first  instance  a  small  portion 
of  the  tail  only  was  visible,  at  right  angles  to  the 
horizon ;  but  night  after  night  as  we  sailed  along, 
it  gradually  became  larger  and  larger,  till  at  last 
up  came  the  head,  or  nucleus,  as  I  ought  properly 
to  call  it.  It  was  a  grand  and  wonderful  sight, 
for  the  comet  now  extended  the  extraordinary 


COMETS.  179 


FIG.  28.— The  Great  Comet  of  1882,  on  October  19  (Artus). 


l8o          THE   STORY   OF   THE   SOLAR   SYSTEM. 

distance  of  one-third  of  the  heavens,  the  nucleus 
being,  perhaps,  about  the  size  of  the  planet 
Venus." — (Vol.  i.,  p.  75.) 

As  regards  Donati's  comet  of  1858,  what  the 
General  says  is  : — 

"  A  very  large  comet  made  its  appearance 
about  this  time,  and  continued  for  several  weeks 
to  be  a  magnificent  object  at  night;  it  was,  how- 
ever, nothing  to  the  one  I  had  seen  in  the  year  1843, 
when  on  the  other  side  of  the  equator." — (Vol.  ii., 
P-  205.) 

Passing  over  the  great  comet  of  1861,  on  which 
I  have  already  said  a  good  deal,  I  must  quit  the 
subject  of  famous  comets  by  a  few  words  about 
that  of  1882,  which,  though  by  no  means  one  of 
the  largest,  was,  in  some  respects,  one  of  the  most 
remarkable  of  modern  times.  It  was  visible  for 
the  long  period  of  nine  months,  and  was  con- 
spicuously prominent  to  the  naked  eye  during 
September,  but  these  facts,  though  note- worthy, 
would  not  have  called  for  particular  remark,  had 
not  the  comet  exhibited  some  special  peculiarities 
which  distinguished  it  from  all  others.  In  the 
first  place,  it  seems  to  have  undergone  certain 
disruptive  changes,  in  virtue  of  which  the  nucleus 
became  split  up  into  four  independent  nuclei. 
Then  the  tail  may  have  been  tubular,  its  cxtrem- 
tiy  being  not  only  bifid,  but  totally  unsymmetrical 
with  respect  to  the  main  part.  The  tubular 
character  of  the  tail  was  suggested  by  Tempel. 
To  other  observers,  this  feature  gave  the  idea  of 
the  comet,  properly  so-called,  being  enclosed  in  a 
cylindrical  envelope,  which  completely  surrounded 
the  comet,  and  overlapped  it  for  a  considerable 
distance  at  both  ends.  Finally  (and  in  this  re- 
sembling Biela's  comet)  the  comet  of  1882  seems 


COMETS.  l8l 

to  have  thrown  off  a  fragment  which  became  an 
independent  body. 

YVhat  has  gone  before,  will,  I  think,  have 
served  abundantly  to  establish  the  position  with 
which  I  started,  namely,  that  comets  occupy  (and 
deservedly  so)  the  front  rank  amongst  those 
astronomical  objects  in"  which,  on  occasions,  the 
general  public  takes  an  interest. 


I  have  thus  completed,  so  far  as  the  space  at 
my  disposal  has  permitted,  a  popular  descriptive 
Survey  of  the  Solar  System.  Those  who  have 
perused  the  preceding  pages,  however  slight  may 
have  been  their  previous  acquaintance  with  the 
Science  of  Astronomy  taken  as  a  whole,  will  have 
no  difficulty  in  realising  that  what  I  have  said 
bears  but  a  small  proportion  to  what  I  have  left 
unsaid.  They  will  equally,  I  hope,  be  able  to  see, 
without  indeed  the  necessity  of  a  suggestion,  that 
all  those  wondrous  orbs  which  we  call  the  planets 
could  neither  have  come  into  existence  nor  have 
been  maintained  in  their  allotted  places  for  so 
many  thousands  of  years,  except  as  the  result  of 
Design  emanating  from  an  All-powerful  Creator. 


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184 


GENERAL   INDEX. 


A. 

Adams,  J.  C.,  145. 

Airy,  Sir  G.  B.,  145. 

Anagram  on  Venus,  68  ;  on  Saturn, 

127. 

Aphelion,  u,  102. 
Apparent  movements  of  the  Planets, 

12,  14. 

Arago,  D.  J.  F.,  99,  104,  140. 
Arctic  regions  of  the  Earth,  75. 
Argelander,  F.  G.  A.,  178. 
Aristarchus  of  Samos,  71. 
Aristarchus  (Lunar  Mountain),  93, 

94- 

Asteroids,  no. 
Aurora   Borealis   and  spots  on  the 

Sun,  55  ;  and  twinkling,  87. 
Autumnal  Equinox,  74. 
Axial  rotation  of  the  Planets,  17, 183. 

B. 

Barnard.  E.  E.,  120,  134. 

Barnard's  Comet,  168. 

Bayeux  Tapestry,  171. 

Beer  and  Madler ;  their  map  of  the 
Moon,  96. 

Belts  on  Jupiter,  115,  123 ;  on  Sat- 
urn, 122. 

Bergeron's  Experiment,  93. 

Bessel,  W.,  124. 

Biela's  Comet,  168. 

Bode's  so-called  "  Law,"  no. 

Bond,  G.  P.,  132,  133. 

Bouvard,  A.,  ii3. 

Bradley,  Rev.  J.,  144. 

Bredechin's  theory  of  Tails  of  Com- 
ets, 157. 

Brorsen's  Comet,  167. 

C. 

44  Canals  "  on  Mars,  106. 
Carpenter,  J.,  134. 


Carrington,  R.  C.,  46,  49. 
Cassini,  J.  D.,  36,  n8,  128,  130,  136. 
Ceres  (Minor  Planet),  112. 
Challis,  Rev.  J.,  146. 
Charts,  Celestial,  112,  147. 
Chinese  observations,  25,  172. 
Clouds  influenced  by  the  Moon,  98. 
Coggia's  Comet,  1874,  151. 
Comets,    150;     periodic,    162,    170; 

remarkable,  176. 

Comparative  sizes  of  the  Planets,  n. 
Comparative  size  of  the  Sun  from 

the  Planets,  17. 
Compression  of  the  Planets,  70,  122, 

137. 
Conjunction  of  the  Planets,  14,  15, 

66.  e 

Copernican  System,  72. 
Copernicus,  72,  73,  76. 

D. 

D' Arrest,  147. 

D' Arrest  s  Comet,  168. 

Dawes,  W.  R.,  106,  133. 

Day  and  night,  12. 

Day,  length  of,  74. 

De  La  Rue,  W.,  35,  36,  49,  52. 

Denning's  Comet,  168. 

Density  of  the  Planets,  17,  183. 

Diameter  of  the  Sun  and  Planets, 

183. 

Di  Vico.  62,  63. 
Di  Vico  s  Comet,  172. 
Distance  of  the   Planets   from   the 

Sun,  20,  183. 

Donati's  Comet  of  1858, 157, 163, 178. 
Dufour  on  Twinkling,  84. 

E. 

Earth,  annual  motion  of,  75  ;  figure 

of,  70. 
Earthshine,  66,  93. 


185 


1 86         THE  STORY  OF  THE  SOLAR  SYSTEM. 


Eccentricity  of  Planetary  orbits,  10, 

^60,  75. 
Eclipses  of  the  Sun,  56,  57  ;  of  the 

Moon,  120. 

Ecliptic,  plane  of,  g,  10,  74. 
Egyptian  System,  72. 
Elliptic  Orbits,  10. 
Elongation  of  Planets,  14. 
Encke,  J.  F.,  147. 
Encke's  Comet,  167,  174,  175. 
Equinoxes,  74. 
Ewart,  Gen.  J.  A.,  178. 

F. 

Faculae  on  the  Sun,  42,  43. 
Faye's  Comet,  168. 
Finlay's  Comet,  168. 
Flammarion,  C.,  62,  66,  171. 
Flamsteed,  Rev.  J.,  140,  144. 

G. 

Galileo,  89,  101,  120,  125. 
Galle,  J.  G.,  147. 
Gassendi,  76. 
Geodesy,  70. 
Georgium  Sidus,  141 
Goldschmidt,  H.,  4. 
Granules,  Solar,  38. 
Gravity   on    the   Sun  and  Planets, 
183. 

H. 

Hall,  A.,  109. 

Halley's  Comet,  164,  171. 

Heat  rays  of  the  Sun,  20  ;  of  the 

Moon,  96. 
Herschel,  Sir  W.,  34,  40,  44,  58,  109, 

103,  134,  137,  138,  142. 
Herschel,  Sir  J.  F.  W.,  24,  96,  99, 

103,  132,  175. 

Hind,  J.  R.,  119,  173,  175. 
Holden,  E.  S.,  106. 
Holland,  Sir  H.,  78. 
Holmes's  Comet,  1892,  159. 
Homer's  Iliad,  cited,  68. 
Hewlett,  Rev.  F.,  47. 
Huggins,  W.,  40. 
Huygens,  C.,  127,  130. 
Hyperbola,  162. 
Hyperbolic  Comets,  162. 

I. 

Inclination  of  planetary  orbits,  9. 
Inferior  Planets,  9,  12. 


J. 

anssen  J.,  47. 

unp  (Minor  Planet),  112. 

upiter :  its  influence  on  Sun-spots 

doubtful,   51,   53 ;    on   Comets, 

171 ;  its  light,  7. 


K. 

Kepler  55,  125,  166. 
Kew  Observatory,  47. 


La  Hire,  63. 
Lalande  34. 

Lassell,  W.,  123,  142,  185. 
Ledger,  Rev.  E.,  84. 
Le  Monnier,  140,  144. 
LeVerner,  U.J.  J.,  147. 
Lexell's  Comet,  172. 
Liais's  Comet,  170. 
Logogriphs  as  to  Venus,  68  ;  as  to 
Saturn,  127. 


M. 

Madler,  J.  H.,  63,  96. 

Magnetism,  Terrestrial,  55. 

Maps,  Astronomical,  112,  147. 

Mars,  loo. 

Maskelyne,  Rev.  N.,  139. 

Mass  of  the  Sun,  and  of  the  Planets, 
20,  183. 

Medium,  Resisting,  175. 

Mercury,  57  ;  phases  of,  15  ;  its  in- 
fluence on  Sun-spots,  51 ;  its 
luminosity,  67. 

Milton,   J.,   Paradise  Lost,   cited, 

Minor  Planets,  no. 
Montigny  on  Twinkling,  84. 
Moon,  80. 

Moonlight,  Brightness  of,  96. 
Motions  of  the  Planets,  10. 
Mountains  of  the  Moon,  90. 

N. 

Napoleon  I.,  154,  176. 
Nasmyth,  J.,  40. 
Needle,  Magnetic,  55. 
Neptune,  143  ;  its  influence  on  Ura- 
nus, 143. 
Newton,  Sir  J.,  176. 


GENERAL   INDEX. 


I87 


Nubian  after-glow,  83. 
Nucleus  of  a  Sun-spot,  22. 

O. 

Obliquity  of  the  ecliptic,  74. 
Occultations  of  Jupiter's  Satellites, 

Olbers/W.,  112,  174. 
Opposition  of  Planets,  13. 
Orbits    of    the    Planets,   9,    10 ;   of 
Comets,  161. 

P. 

Pallas  (Minor  Planet),  112,  113,  114. 

Parabola,  162. 

Penumbra  of  a  Sun-spot,  22. 

Perihelion,  IT,  102. 

Periodic  Comets,  162,  170. 

Perturbations  of  Uranus  by  Nep- 
tune, 145. 

Phases  of  an  inferior  Planet,  15  ;  of 
a  superior  Planet,  16  ;  of  Mars, 
loo-  of  Jupiter,  115. 

Piazzi,  G.,  in. 

Planets,  classification  of,  7  ;  com- 
parative sizes  of,  ii  ;  move- 
ments of,  10. 

Plutarch,  71. 

Poles  of  Mars,  snow  at,  104. 

Primary  Planets,  7. 

Ptolemaic  System,  72. 

Q. 

8uadrature  of  a  Planet,  16. 
uenisset's  Comet,  1893,  156. 

R. 

Rays  of  the  Sun,  20. 
Refraction,  77  ;  effect  of,  77. 
Red  soot  on  Jupiter,  118. 
Resisting  Medium,  175. 
Retrograde  motion  of  a  Planet,  15. 
"  Rice  grains  "  on  the  Sun,  39,  40. 
Rings  of  Saturn,  128. 
Rosse,  Earl  of,  93. 
Rotation  of  the  Sun,  24,  28  ;  of  the 

planets,  17,  183. 
Rotundity  of  the  Earth,  76. 

S. 

Satellites  of  Mars,  16,  184  ;  of  Jupi- 
ter, 16,  184  ;  of  Saturn,  16,  185  ; 


of  Uranus,  16, 185  ;  of  Neptune, 
16,  185. 

Saturn,  122. 

Sawerthai's  Comet,  165. 

Schiaparelli,  J.  V.,  59,  62,  63,  106. 

Schmidt,  J.  F.  F.,  96. 

Schroter,  J.  J.,  58,  62,  63. 

Schwabe's  observations  of  Sun- 
spots,  46. 

Seas,  Lunar,  94. 

Seasons  on  the  Earth,  54,  74. 

Secchi,  A.,  28,  35,  42,  50,  84,  86. 
106. 

Secondary  Planets,  8. 

Shakespeare,  citations  from,  152. 

Smyth,  C.  P.,  92. 

Snow  on  Venus,  66  ;  on  Mars,  104  ; 
doubtful  on  Saturn,  123. 

Solstices,  74. 

Spherical  form  of  the  Earth,  70. 

Sporer,  46,  50. 

Spots  on  the  Sun,  21,  n8  ;  on  Ve- 
nus, 61  ;  on  Jupiter,  117 ;  on 
Saturn,  123. 

Stewart,  B.,  52,  56. 

Struve,  O.,  134,  142. 

Sun,  18,  183. 

Superior  Planets,  8  ;  movements  of, 
J5- 

Surfaces  of  the  Sun  and  Planets, 
.183. 

Swift's  Comet,  168. 

Systems  of  the  Universe,  72. 

T. 

Tables  of  the  Major  Planets,  183. 

Tails  of  Comets,  154. 

Temple's  Periodical  Comets,  167. 

Terminator  of  the  Moon,  90. 

Thomson's  Seasons  cited,  153. 

Tides,  98. 

Total  Eclipses  of  the  Sun,  57. 

Transits  of  inferior  Planets,  13,  25, 

65- 

Trouvelot,  133. 

Tuttle's  Periodical  Comet,  167,  168. 
Twilight,  80. 
Twinkling,  83. 

Tycho  (Lunar  Mountains),  94. 
Tycho  Brahe,  73. 
Tychonic  System,  73. 

U. 

Umbra  of  a  Sun-spot,  22. 
Uranus,  138  ;  the  influence  of  Nep- 
tune on,  143. 


THE   STORY  OF  THE   SOLAR  SYSTEM. 


V. 

Velocity  of  the  planets,  183. 

Venus,  61  ;  phases  of,  15  ;  its  influ- 
ence on  Sun-spots,  51,  52,  53. 

Vernal  Equinox,  74. 

Vesta  (minor  planet),  112,  113. 

Virgil,  citation  from,  50. 

Volume  of  the  Sun,  20 ;  of  the 
Planets,  183. 

W. 

Weather  influences  ascribed  to  the 
Sun,  44 ;  to  the  Moon,  98. 


"  Willow  leaves  "  on  the  Sun,  39,  40. 
Wilson's  theory  of  Sun-spots,  32. 
Winnecke's  Periodical  Comet,  167. 
Wolf,  R.,  observer  of  Sun-spots,  45, 

48. 
Wolf's  Periodical  Comet,  168. 

Y. 

Young,  C.  A.,  131. 

Z. 

Zodiac,  movement  of  Planets  in,  15. 


(7) 


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